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Canadian Journal of Pain logoLink to Canadian Journal of Pain
. 2026 Jan 29;10(1):2598285. doi: 10.1080/24740527.2025.2598285

Ambient green light reduces knee pain in osteoarthritis patients

Melissa S O’Brien a, Sharon Amey a, Chris DeBow a, Karim Mukhida a, Jason J McDougall a,b,✉
PMCID: PMC12867406  PMID: 41640822

ABSTRACT

Background

Regular viewing of dim green light has been shown to reduce the pain associated with migraine, fibromyalgia, and post-surgery.

Aim

The present study examined whether visual exposure to ambient green light could also alleviate osteoarthritis (OA) pain.

Methods

Nineteen patients diagnosed with moderate to severe knee OA pain were exposed to dim white light (6.57 ± 1.00 lux) for 1–2 hours per day for 10 weeks. Following a 2-week wash-out period, patients then received green light treatment (wavelength = 525 nm, 6.82 ± 0.78 lux) for 1–2 hours per day for a further 10 weeks. The primary outcome measure was changes in arthritis disability score as measured by the Western Ontario and McMaster University Arthritis Index (WOMAC) questionnaire. Secondary outcomes included patient-reported changes in pain intensity, pain disability, and patient global satisfaction.

Results

With green light therapy (GLT), average WOMAC scores decreased from 44.1 ± 17.5 at baseline to 32.5 ± 16.2 whereas white light therapy (WLT) had no significant effect on arthritis disability scores (39.6 ± 15.3). While both WLT and GLT reduced pain intensity, the analgesic effect of green light was significantly greater. Pain interference scores were lower following GLT whereas WLT had no effect on this parameter. The Patient’s Global Impression of Change was significantly improved over baseline with both WLT and GLT.

Conclusion

Daily exposure to ambient green light had a beneficial effect on OA knee pain. The neural pathways between the visual system and central pain modulation regions requires further investigation.

KEYWORDS: Pain, osteoarthritis, green light, adjunct therapy, photobiomodulation

Introduction

The latest study on the Global Burden of Disease reported that almost 600 million people currently live with osteoarthritis (OA) worldwide and this figure is expected to double by 2050.1 Approximately 60% of OA patients are women who are at a higher risk of developing the disease because of sex-differences in joint architecture, biomechanics, and hormones.2 The majority of individuals living with OA experience moderate to high levels of joint pain which significantly impacts quality of life. Pharmacological management of joint pain involves long-term use of non-steroidal anti-inflammatory drugs (NSAIDs), intraarticular corticosteroid injections, serotonin and norepinephrine reuptake inhibitors, or opioids.3 Each of these medications is associated with significant negative side-effects including gastrointestinal ulceration, renal damage, and nausea. Furthermore, patients report inadequate pain relief with these pharmacological approaches and compliance can be an issue with fears relating to side-effects and analgesic overuse.4

Non-pharmacological strategies are a vital component of an OA pain management plan. Recommendations include regular low impact exercise, cognitive-based therapies, and a healthy diet which have all been shown to help patients manage their joint pain.5–7 Use of non-pharmacological approaches as an adjunct to traditional drug therapies is attractive as they tend to be safe and easy to implement. The main barriers to non-pharmacological treatments for pain are high costs and poor access to the tools.8 Therefore, the development of affordable and accessible non-pharmacological pain management approaches is vital for their adoption by patients and healthcare providers. An emerging treatment that may be beneficial in treating OA pain is green light therapy (GLT). Exposure to low-intensity green light has been found to reduce pain and photophobia acutely in migraine patients9 and curtail anxiety in patients undergoing elective surgery.10 Chronic pain associated with a variety of disorders can also be alleviated by viewing green light daily for short periods.11,12 In migraine patients, for example, daily exposure to green light for 1–2 hours per day has been shown to reduce both the frequency and intensity of migraine attacks, when compared to control white light treatment.12 GLT administered either by ambient light emitting diodes (LEDs) or the use of filtered spectacle lenses has also been found to reduce pain intensity and opioid intake in fibromyalgia patients while at the same time providing a small but significant improvement in quality-of-life.11,13

With respect to OA, a recent preclinical study in rodents reported a reduction in pain-like behavior following a regimen of ambient green light exposure which involved the release of endogenous analgesic lipids.14 Electrophysiological recording from knee joint afferents indicated that the green light was not acting in the periphery but rather a central mode of action was occurring, likely via the visual system. The mechanism by which GLT produces analgesia is still unclear. Neural tracts originating in the retina terminate in the ventral lateral geniculate nucleus and the intergeniculate leaflet of the periaqueductal gray and bright light can inhibit GABAergic neurones in these regions leading to a reduction in nocifensive behavior.15 Furthermore, low intensity green light activates this pathway leading to a reduction in inflammatory joint pain.16 The aim of the present study was to determine if GLT could be beneficial in the alleviation of OA pain in a cohort of OA patients.

Methods

Study participants

All protocols received prior ethics board approval from Nova Scotia Health Authority (REB-1028226) and the study was registered with Clinicaltrials.Gov (NCT-05398666) Participants were recruited using paper advertisements displayed in local hospitals, physiotherapy clinics, recreation facilities and public libraries in Halifax, Nova Scotia, Canada between February 2023 and February 2024. Adults (18 years of age or older) meeting the following inclusion/exclusion criteria were recruited to participate in the study:

Inclusion Criteria:

  • Osteoarthritis of the knee according to American College of Rheumatology criteria.17

  • Moderate to severe pain, as defined by an average 7-day pain score greater than 4.0 on an 11-point numerical rating scale for pain intensity (NRS-PI), collected for 4 weeks.

  • Stable pain medication use for 14 days prior to initiation of intervention.

  • Ability to follow the protocol with reference to cognitive and situational factors (e.g., stable housing).

  • Ability to read and write English.

  • Willing and able to give informed consent.

Exclusion Criteria:

  • Currently enrolled in other clinical trials involving a pharmaceutical treatment.

  • Arthroscopic knee surgery scheduled within 8 months of study initiation.

Study protocol

Verbal consent was discussed with participants by telephone. During the telephone discussion, the coordinator provided a summary of the study, invited questions, and asked potential participants if they would like to proceed to an informed consent discussion. At that point, the research coordinator read the consent form to potential participants verbatim. Included in the consent form is a section explaining that participants may withdraw from the study at any point by contacting the coordinator. After reading the consent form, the coordinator invited and addressed questions, then sought and documented verbal consent. Immediately following the telephone consent discussion, the research coordinator emailed (or mailed for the few participants who did not use e-mail) a copy of the consent form which was again read to the participant verbatim at follow-up. Participants were not asked to return a copy of the consent form; however, the coordinator telephoned participants to confirm receipt of the consent form and invite any additional questions.

Following recruitment to the study, participants were asked to report four weekly NRS-PI pain scores, the mean of which was used to determine study eligibility. Participants were subsequently provided with the first strip of white LEDs for use in arm one of the study. Participants were asked to use the lights for a total of 10 weeks and provided with the following instructions to describe the use of lights and potential activities to carry out during light treatment:

  • Use LED as light source for 1–2 hours/day without external ambient light.

  • LEDs should not be directed toward the affected knee but rather illuminate the room.

  • Remain within 1–2 meters from LED and keep eyes open (i.e. do not sleep), but avoid staring directly at LED light or using devices with additional light sources (e.g. cell phone, computer screen, television).

  • Participants were free to carry out normal daily activities such as reading, knitting, cooking, etc.

Following a subsequent minimum two-week washout period with no light treatment, a set of green LEDs were issued to each participant. The same instructions were provided for use of green lights. Participants were not informed which LED strip were active or control lights.

Light source

White (#LS-AC60-66-WH; 9.6 watts, 120 volts, 120-degree beam angle) and green (#LS-AC60-6-GR; 525-nm wavelength, 8 watts, 120 volts, 120-degree beam angle) LED strips were procured for this study from an online vendor (LEDsupply.com, Randolph, VT, USA). LED strips were independently examined by light experts for safety (Dalhousie University Department of Biophysics). To ensure constant dim light exposure, the LED strips were modified using heat-resistant electrical tape to cover approximately 90% of lights. Light intensity was measured using a digital luxmeter (LX1330B; Amazon Canada, Toronto, ON, Canada) to confirm lux range (white light: 6.57 ± 1.00 lux; green light: 6.82 ± 0.78 lux, measured at a distance of 1 meter).

Outcome measures

In accordance with the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials,18 our study consisted of outcome measures in several core domains:

Primary Outcomes:

  • Changes in Arthritis Disability score as measured via the Western Ontario and McMaster University Arthritis Index (WOMAC questionnaire). WOMAC scores were measured at baseline and after each light phase of the study.

Secondary Outcomes:

  • Changes in reported pain measured using an 11-point NRS-PI. NRS-PI scores were collected weekly during the four-week baseline, and daily throughout the light phases of the study.

  • Changes in reported pain measured using the Brief Pain Inventory Short Form (BPI). BPI questionnaires were completed at baseline and after each light phase of the study.

  • Changes in patient satisfaction measured using the Patient’s Global Impression of Change (PGIC) questionnaire which were completed following each light phase of the study.

Study participants were also asked to report any adverse events experienced across the duration of the study and any change in medication use or non-pharmacological pain management strategies adopted. As per participant preference, questionnaire data were collected via paper forms (distributed as a daily diary booklet at the beginning of each 10-week phase) or electronically (as a daily survey delivered to their e-mail) using RedCap software19 hosted at Nova Scotia Health.

Statistical analysis

Sample size calculations were based on effect sizes reported in previous studies.11,12 Only data from participants that completed both treatment arms of this study were analyzed and reported. All data were plotted as mean ± S.E.M. and tested for normality using the Kolmogorov-Smirnov test. Primary outcome data (change in mean WOMAC scores) and the secondary outcome measure of mean BPI scores were normally distributed and analyzed using a repeated measures one-way analysis of variance (ANOVA) with Tukey’s post hoc test. The overall change in weekly NRS-PI scores across a study arm was calculated for each light group by subtracting baseline values and data were tested by the non-parametric Wilcoxon signed-rank test. Time-course data were calculated as the percent change in NRS-PI values across the study relative to baseline and were tested using a mixed-effects analysis with a post hoc Sidak’s multiple comparison test. The PGIC survey (non-parametric data) was analyzed using the Wilcoxon signed rank test. A p value < 0.05 was the threshold for statistical significance. All analyses were conducted using PRISM version 8.4.0 (GraphPad Software, MA, USA).

Results

Enrollment and participant demographics

In total, 47 individuals were screened for study eligibility. Seven failed the initial eligibility criterion of moderate to severe pain, as defined by an average pain score greater than 4.0, and 40 participants were enrolled in the study. Of these 40 participants, 21 subsequently withdrew from the study for medical or personal reasons and were excluded from data analysis. Nineteen participants completed both arms of the study (Figure 1). Most study participants identified as female or women (68%) and white (90%) with an average age of 61 ± 8.1 (range: 52–77) at the time of enrollment. Mean baseline pain scores were 6.3 ± 1.6 (range: 4.3–9.5) measured using the NRS-PI.

Figure 1.

Figure 1.

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Flowchart of participant recruitment and study protocol. Of the 47 OA patients recruited to the study, 19 completed both arms of the phototherapy. Using a one-arm crossover design, participants were initially exposed to white light (1–2 hours/day for 10 weeks) followed by a washout period (2 weeks), and finally exposure to ambient green light (1–2 hours/day for 10 weeks).

Eighty-five percent of participants reported actively using medication for OA pain management. The most commonly used medications were oral NSAIDs/acetaminophen (75%), or topical NSAIDs (25%), while 10% of participants reported using either gabapentinoids, cannabis, or steroid injections to manage their pain. Non-pharmacological pain management and exercise were also commonly used by study participants. Participant demographics and baseline pain data are summarized in Table 1.

Table 1.

Summary of osteoarthritis patient demographics and baseline pain data.

Participant demographics
Average age, years ± SD (range) 61 ± 8.1 (52–77)
Sex Female, 68.4%
Male, 26.3%
Did not disclose, 5.3%
Gender Woman, 68.4%
Man, 26.3%
Did not disclose, 5.3%
Self-identified race, % Black, 5.3%
Did not disclose, 5.3%
White, 89.5%
Baseline pain score, mean NPS ± SD 6.3 ± 1.6 (4.25–9.5)
Currently Using Medication for OA pain management (%) 84.20%
NSAIDs or Acetaminophen 73.70%
Topical NSAID 26.30%
Steroid Injection 10.50%
Gabapentinoids 10.50%
Cannabis 10.50%
Currently using non-pharmacological approaches for OA pain management 57.90%
TENS 15.80%
Massage Therapy 15.80%
Physiotherapy 10.50%
Exercise for OA pain management 63.20%
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Light exposure and side effects

Participants reported using white LEDs for 69 ± 25 minutes per day and green LEDs for 73 ± 29 minutes per day. No adverse events were reported with use of either white or green lights.

Primary outcome measure

Arthritis disability scores were measured at baseline and following each treatment arm using the WOMAC questionnaire. The total WOMAC score at baseline was 44.1 ± 17.5 (range: 13–70) which was significantly reduced following GLT (32.5 ± 16.2, range: 10–76, p < 0.05 one-way ANOVA with Tukey’s post hoc test, Figure 2A). Exposure to white light therapy (WLT), in comparison, did not affect the total WOMAC score relative to baseline (39.6 ± 15.3, range: 17 to 76, p > 0.05 one-way ANOVA with Tukey’s post hoc test, Figure 2A). WLT total WOMAC scores did not differ from GLT (p > 0.05 one-way ANOVA with Tukey’s post hoc test, Figure 2A). The mean difference in total WOMAC score between WLT and GLT was 7.53 ± 12.7.

Figure 2.

Figure 2.

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Comparison of white versus green light exposure on Western Ontario and McMaster University Arthritis Index (WOMAC) scores. The total WOMAC score was significantly reduced by green light treatment whereas white light had no effect (A). The WOMAC pain subcategory was reduced by both light treatments (B); however, the effect of green light was greater. Phototherapy had no effect on WOMAC joint stiffness (C) or joint function (D). Data are shown as means ± S.E.M and comparisons were made by one-way ANOVA with Tukey’s post hoc test.

The effect of WLT and GLT on pain, stiffness, and physical function scores were also compared with each other and to baseline measurements. Relative to baseline, treatment with green light improved pain scores but had no effect on stiffness or function associated with knee OA (Figure 2B–D, one-way ANOVA with Tukey’s post hoc test). Exposure to white light improved pain scores relative to baseline but did not affect either stiffness or function as measured using the WOMAC questionnaire (Figure 2B–D, one-way ANOVA with Tukey’s post hoc test). Comparing WLT with GLT, only stiffness measures were significantly different (Figure 2C, p < 0.05, one-way ANOVA with Tukey’s post hoc test). The mean difference between light treatment arms in WOMAC pain was 0.63 ± 2.4, WOMAC stiffness 0.74 ± 2.0, and WOMAC function 5.05 ± 9.7.

The effect of WLT and GLT on pain, stiffness, and physical function scores were also compared to baseline measurements. Relative to baseline, treatment with green light improved pain scores but had no effect on stiffness or function associated with knee OA (Figure 2B–D, one-way ANOVA with Tukey’s post hoc test). Exposure to white light improved pain scores relative to baseline but did not affect either stiffness or function as measured using the WOMAC questionnaire (Figure 2B–D, one-way ANOVA with Tukey’s post hoc test). Comparing WLT with GLT, only stiffness measures were significantly different (Figure 2C, p < 0.05, one-way ANOVA with Tukey’s post hoc test). The mean difference between light treatment arms in WOMAC pain was 0.63 ± 2.4, WOMAC stiffness 0.74 ± 2.0, and WOMAC function 5.05 ± 9.7.

Secondary outcome measures

Pain measurements

Pain was also assessed using the BPI which measures pain severity and interference to calculate a summative pain score. The average summative BPI score at baseline was 10.4 ± 4.2 (range: 2.8–17.0). Relative to baseline, significant reductions in total BPI score following 10 weeks of exposure to GLT were observed (6.6 ± 4.2, range: 0.87–17.4, p < 0.05, one-way ANOVA with Tukey’s post hoc test Figure 3A). WLT did not affect this parameter relative to the baseline measurements (8.4 ± 4.4, range: 0.8–17.2, p > 0.05, one-way ANOVA with Tukey’s post hoc test Figure 3A) and was significantly different from GLT (p < 0.05). The mean difference in total BPI scores between WLT versus GLT was 1.83 ± 2.5.

Figure 3.

Figure 3.

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Effect of green and white light treatment on the Brief Pain Inventory (BPI) scores. Visual exposure to green light significantly reduced BPI total (A), severity (B), and interference (C) scores whereas white light did not Data are shown as means ± S.E.M. and comparisons were made by one-way ANOVA with Tukey’s post hoc test.

Examination of pain severity and interference subscores also revealed reductions following GLT. Baseline pain severity (5.1 ± 2.0, range: 2.5–8.8) reduced to 3.4 ± 2.2 (range: 0.0–8.8) with GLT (p < 0.05, one-way ANOVA with Tukey’s post hoc test). WLT did not affect BPI pain severity relative to baseline measures (4.3 ± 2.0: range: 0.5–8.3, p > 0.05) but did differ from GLT (p < 0.01, one-way ANOVA with Tukey’s post hoc test, Figure 3B). Examination of pain interference revealed that treatment with GLT also resulted in a significant reduction in these scores relative to baseline and WLT (p < 0.05, Figure 3C), while WLT alone did not differ from baseline (p > 0.05 one-way ANOVA with Tukey’s post hoc test, Figure 3C). The mean difference in pain severity with WLT versus GLT was 0.86 ± 1.4 and for pain interference it was 0.97 ± 1.5.

GLT significantly improved pain intensity as measured by the NRS-PI compared to WLT (Wilcoxon signed-rank test, p < 0.05, Figure 4A). However, longitudinal analysis of weekly NRS-PI scores did not reveal a significant effect of light treatment over the 10-weeks of exposure (p > 0.05, mixed-effects analysis with Sidak’s multiple comparisons test, Figure 4B).

Figure 4.

Figure 4.

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Changes in Numerical Rating Scale for Pain Intensity (NRS-PI) following green and white light. Green light reduced NRS-PI scores more than white light overall (A). The analgesic effect of green light became more prominent over the 10 weeks of phototherapy (B). Means ± S.E.M. Overall effect was assessed using the Wilcoxon signed-rank test and the time course was tested by a mixed-effects analysis.

Global impression of change

Patient satisfaction with light treatment was measured using the PGIC questionnaire. Overall PGIC scores were not significantly different between GLT and WLT arms (p > 0.05, paired Student’s t-test, Figure 5A). The rankings of participant satisfaction from “very much worse” to “very much improved” for each domain (overall health, physical activities, social activities, work activities, mood, and pain) are summarized in Figure 5B–G. Of note, approximately 84% of participants reported improvements in overall health following GLT, compared with 52% following WLT. A positive effect of light therapy on mood was reported by 40% with green light use and 26% with white light but worsening of mood was also reported by some participants in each arm of light therapy. In certain domains, a minimal impact of light therapy was reported. For example, when asked about the impact on social activities, the majority of participants reported no change (WLT: 74%, GLT: 84%).

Figure 5.

Figure 5.

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Effect of green versus white light on patient satisfaction measured using the Patient’s Global Impression of Change (PGIC) questionnaire. There was no overall difference in PGIC scores between green and white light (A). Individual rankings for overall health (B), physical activity (C), social activity (D), work activity (E), mood (F), and pain (G) are also shown. Patient satisfaction was better with green light compared to white light for all PGIC scores except social activities which was unaffected by phototherapy. Data for the overall PGIC score are presented as means ± S.E.M. and tested by the Wilcoxon signed-rank test.

Participant reported changes in pain and pain management with light use

At the end of each treatment arm, participants were asked to report changes in both medication and treatments directed toward their knee pain. Five participants changed their medication regimen during the white light phase. Four of these increased their consumption of analgesics while one required less NSAIDs. Of the four that increased their consumption, one began a new medication and one increased the dose of an existing treatment for knee pain. Two others began medication and dietary supplements to treat sleep disturbances related to knee pain. Two participants changed their pain management treatments during the white light phase. One incorporated acupuncture and chiropractic treatment while the other participant ceased knee massage and formal exercise.

Five participants changed their medication regimen during the green light phase. Two reported corticosteroid injections in the knee and another increased over the counter pain medication. Two others reported decreasing or eliminating over the counter pain relief. Only one participant reported adopting new non-pharmacological treatments, which was a combined physiotherapy and weight-loss program.

As a final survey question following each treatment arm, participants were invited to provide additional comments on their experience with light therapy. Twelve participants used this opportunity to elaborate on changes in pain. Three reported positive effects from both the white and green light. In response to green light use, ten reported a general reduction in pain and stiffness, and that they had reduced need for mobility aids.

Discussion

While pharmaceutical agents can be an effective means of controlling joint pain, non-pharmacological approaches provide an important adjunct to help complement traditional analgesics. Regular viewing of dim green light has shown promising results clinically to reduce the severity of ongoing pain associated with migraine, headache, and fibromyalgia.11 The data presented here show for the first time that GLT can also alleviate OA pain and improve the quality of life of patients.

The primary outcome measure for this one-way crossover trial was changes in WOMAC scores as this sensitive test encompasses OA symptoms and physical functional. Participants reported using GLT for approximately one hour per day and attained a significant decrease in total WOMAC score whereas WLT had no overall effect. This reduction in total WOMAC score did not meet the minimum clinically important difference commonly referenced for non-surgical knee OA interventions20 suggesting that GLT may be best suited as an adjunct therapy. Further examination of the WOMAC subcategories revealed that GLT significantly reduced pain but had no effect on joint stiffness or function. The involvement of a placebo effect cannot be overlooked as participants were requested to view the lights in an environment free from other light sources such as cell phones and television. Indeed, WOMAC pain scores were also reduced during the dim white light phase similar to GLT suggesting that a calm, meditative state may have been achieved which is known to improve OA pain symptoms.21 Nevertheless, using the secondary outcome measure of BPI to assess pain severity and interference, GLT attenuated pain severity whereas WLT did not. Further examination of the pain interference subscores confirmed green light to be an effective treatment strategy compared to white light.

The analgesic response to GLT on OA pain described here is consistent with its effect in other chronic pain conditions. Clinical trials with migraine and fibromyalgia patients reported reductions in pain using NRS-PI in addition to disease specific measures including the Headache-Impact Test-6 and the Fibromyalgia Impact Questionnaire.11,12 In these studies, migraine patients reported a 60% reduction in NRS-PI scores and fibromyalgia patients reported approximately a 40% reduction following 10 weeks of GLT. A similar reduction is reported here, where OA patients experienced a 33% reduction in pain measured using NRS-PI. Disease specific questionnaires also revealed significant improvements in social and physical domains in response to GLT, consistent with the findings presented here using the WOMAC questionnaire. Improvements in general health and wellbeing are also important measures to assess the utility of pharmacological and non-pharmacological treatments for pain. Following phototherapy with green light, 84% of participants in the present study reported improvements in their overall health, compared with 53% following white light treatment. Comparable positive responses to overall health have been measured in previous trials; however, significant improvements with both white and green light use have been reported in all clinical studies to date.

The mechanism of action for GLT-induced analgesia is still unclear but appears to involve central pathways via the visual system. A direct peripheral effect of GLT on joint nociceptors is unlikely as it has been reported that ambient exposure to green light has no effect on joint afferent mechanonociception in a rat model of OA.14 Ibrahim et al. showed in rats that green light permissive contact lenses produced antinociception while opaque contact lenses prevented GLT-induced analgesia.22 In fibromyalgia patients, wearing green tinted spectacles for 4 hours per day for 2 weeks resulted in a clinically significant reduction in opioid intake for pain management.13 These studies suggest that green light imparts its analgesic effect by entering the visual system yet the neural pathways between the eye and pain modulatory centers in the brain require further investigation. Disruption in projections from the retina to the ventrolateral geniculate nucleus blocked the analgesic effect of GLT in a mouse model of inflammatory joint disease,16 while microinjection of lidocaine into the rostroventral medulla inhibited GLT antinociception in response to a noxious thermal stimulus.22 Furthermore, neuropharmacological experiments indicate that GLT stimulates the release of endogenous opioids and endocannabinoids which both attenuate pain signaling in the dorsal horn of the spinal cord.14,16,23 Since brain regions that modulate spinal pain circuits are compromised in OA patients,24,25 we postulate that visual exposure to green light may restore descending inhibitory control mechanisms; however, further research is needed to test this hypothesis.

A number of limitations are acknowledged in the current study. Due to the exploratory nature and protracted duration of the treatment, a relatively small number of participants completed both arms of the phototherapy. Participants were largely homogeneous, consisting mostly of older, white women/females and the study was therefore underpowered to investigate potential sex- or gender differences with green light use. Nevertheless, the encouraging results reported here merit further study in a larger, more diverse population of OA patients. Effective blinding may also have been an issue in the present study. All participants were blinded to the active and control lights by the study coordinator; however, the use of colored light for medical purposes is well known and participants may have independently investigated their use for pain. Participants also may have had preconceived beliefs about which light was the active intervention, but none of the participants disclosed this possibility. These shortcomings in reasonable blinding could have led to a placebo effect in some participants. Unlike drug studies in which a placebo control group receives an inert compound, blinding to different wavelengths of light is not possible and is a known limitation of photobiomodulation studies.26 The light intensity, exposure time, and frequency for GLT phototherapy still need to be optimized clinically and remain a limitation of these types of studies. The present investigation used similar methods to previous clinical trials, but the physical parameters required to impart analgesia with green light have yet to be confirmed. It is also unknown how long GLT-induced analgesia persists following light treatment cessation although preclinical studies suggest that pain relief could continue to last for days.22,23,27 For this reason, a one-way crossover trial where participants completed the white light arm prior to entering the green light arm was warranted in this pilot study to avoid potential green-light effects extending into the control white light phase. This, however, means that participants were not randomized at baseline and we cannot rule out crossover effects or regression to the mean in our study results. Further research that focuses on mapping treatment effects to exposure time is needed which would allow for GLT to be investigated using a randomized controlled trial paradigm as well as reducing the time burden on patients wishing to incorporate phototherapy as an adjunct pain management strategy.

Conclusion

In summary, the present study showed for the first time that daily viewing of dim green light reduced joint pain in OA patients. No adverse effects were reported and patients’ overall health improved with green light phototherapy. As such, GLT could be a safe, effective, and affordable adjunct for the long-term management of chronic OA pain.

Acknowledgments

The authors are extremely grateful to the participants involved in this study. Original data are available upon request.

Funding Statement

The work was supported by a project grant from The Arthritis Society of Canada [Grant # 21-0000000024].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data are available upon request.

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Associated Data

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Data Availability Statement

Data are available upon request.

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