Tolerability of long‐term cannabidiol supplementation to healthy adult dogs

Abstract

Background

Cannabidiol (CBD) has therapeutic potential in companion animals. Shorter‐term studies have determined that CBD is well tolerated in dogs with mild adverse effects and an increase in alkaline phosphatase (ALP) activity. There is need to assess CBD's long‐term tolerability.

Hypothesis

Determine the long‐term tolerability of CBD administered PO to healthy dogs for 36 weeks at dosages of 5 and 10 mg/kg body weight (BW)/day. Our hypothesis was that CBD would be well tolerated by dogs.

Methods

Eighteen healthy adult beagle dogs were randomly assigned to 3 groups of 6 each that received 0, 5, or 10 mg/kg BW/day CBD PO. Dogs were adapted to their housing for 3 weeks and received treatment for 36 weeks once daily with food. Adverse events (AEs) were recorded daily. Blood biochemistry profiles were monitored every 4 weeks. Data were analyzed as repeated measures over time using a mixed model, with significance at α = 0.05.

Results

The 0 and 5 mg/kg treatment groups had similar fecal scores, and the 10 mg/kg treatment group had higher frequency of soft feces. No other significant AEs were noted. An increase (P < .0001) in ALP activity occurred in groups that received CBD. Remaining blood variables were within reference range.

Conclusions and Clinical Importance

Chronic administration of CBD in healthy dogs at 5 mg/kg was better tolerated than 10 mg/kg, and both dosages caused an increase in ALP activity. Although our data does not indicate hepatic damage, it is recommended to monitor liver function in dogs receiving CBD chronically.

Abbreviations

5‐HT_1A and 5‐HT_3A

5‐beta hydroxytryptamine receptor 1 and 3

AE

adverse event

ALP

alkaline phosphatase

ALT

alkaline aminotransferase

AST

aspartate aminotransferase

BFI

body fat index

BW

body weight

CBD

cannabidiol

FAAH

fatty acid amide hydrolase

GGT

gama‐glutamyl transferase

GLIMMIX

generalized linear mixed model

GPR55

G protein‐coupled receptor 55

MCT

medium‐chain triglyceride

PC

phytocannabinoid

PPARγ

peroxisome proliferator‐activated receptor gamma

THC

delta‐9‐tetrahydrocannabinol

TRPV1

transient receptor potential vanilloid 1

1. INTRODUCTION

Cannabidiol (CBD) is the most studied non‐psychotropic phytocannabinoid (PC) among the more than 150 PCs found in hemp. ¹ Cannabidiol aids in treating inflammatory and neurological conditions, and is commonly used for its calming potential. ² , ³ The majority of CBD research has utilized rodents, humans, or in vitro models, and scientific literature on dogs and cats is beginning to emerge. ⁴ Cannabidiol is a non‐competitive allosteric modulator of the G protein‐coupled cannabinoid receptor 1 (CB1) and an inverse agonist of CB2, ⁵ , ⁶ key components of the endocannabinoid system. Whereas CB1 is mostly concentrated in the central nervous system and regulates neurotransmitter release, CB2 is mainly located in cells of the immune system (intestine, tonsils, spleen) and plays a role in controlling cytokine release. ⁵ , ⁶ , ⁷ The negative modulation of these receptors by CBD aids in homeostasis and attenuation of inflammation. Other receptors that CBD has been found to interact with in mammals include fatty acid hydrolase (FAAH; inhibits enzyme hydrolysis of anandamide), G‐protein coupled receptor 55 (GPR55; bone metabolism), transient receptor potential vanilloid 1 (TRPV1; analgesic and antiepileptic), peroxisome proliferator‐activated receptor gamma (PPARγ; anti‐inflammatory and antioxidant), and serotonin 5‐beta hydroxytryptamine receptors 1 and 3 (5‐HT_1A and 5‐HT_3A; antidepressant and anxiolytic). ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹²

The 2018 Farm Bill moved hemp (<0.3% delta‐9‐tetrahydrocannabinol [THC]) from schedule I to II drug under the Controlled Substances Act, facilitating CBD research in dogs. The Drug Enforcement Administration (DEA) also reclassified Epidiolex, a purified CBD solution, for PO use as a treatment option for epilepsy in children, as a schedule V drug in 2018. ¹³ Although CBD has low potential for abuse in humans, currently, it is not generally recognized as safe (GRAS) as a food additive by the Food and Drugs Administration (FDA), and thus its tolerability and safety profile in dogs have not been fully studied. Short‐term studies in dogs found that CBD supplemented at 4 to 20 mg/kg of body weight (BW) per day for 6 to 12 weeks was well‐tolerated, except for increased alkaline phosphatase (ALP) activity in most dogs. Administering CBD to dogs at incremental dosages up to 64.7 mg/kg resulted in only mild AEs, whereas those given THC or a combination of CBD and THC had observable AEs. ¹⁴

Long‐term safety of CBD had not been explored until recently. ¹⁵ It was concluded that CBD was well‐tolerated by dogs at 4 mg/kg once daily for 6.5 months but caused an increase in ALP activity. ¹⁵ More research is needed to elucidate CBD's tolerability long‐term, including exploring various dosages or formulas for longer term. Our goal was to determine the tolerability of CBD‐administered PO to healthy adult dogs for 36 weeks at 2 dosages (5 and 10 mg/kg BW/day). Our hypothesis was that CBD would be well‐tolerated at the tested dosages, only causing increased ALP activity without indication of hepatic dysfunction or other signs of intolerance.

2. MATERIALS AND METHODS

2.1. Treatments

An industrial hemp broad spectrum extract with 94.5% CBD, 4.6% cannabigerol (CBG), 0.3% cannabidivarin (CBDV), and 0.6% cannabichromene (CBCH) was formulated to contain 50 and 100 mg/mL CBD in a medium‐chain triglyceride (MCT) vehicle oil (KND Labs, LLC; Lakewood Colorado) to be administered at 5 and 10 mg/kg CBD once daily, respectively. The placebo formula contained MCT oil with no hemp extract. The final treatment oils had <0.3% terpenes and THC was non‐detectable (below 0.01% limit of quantitation [LOQ]; SC Labs, Denver CO, U.S.A.). A quality control panel of heavy metals, mycotoxins, residual solvents, and pesticides was evaluated (SC Labs, Denver, Colorado), and no toxins were detected. All research personnel, except the co‐principal investigator, were blinded to the group assignment. The CBD concentration for both the 50 and 100 mg/mL oils was tested at the beginning and at end of the study (week 36) and found to be stable, with no differences observed.

2.2. Animals and study design

Eighteen adult healthy research beagles (9 neutered male, 9 spayed female) were enrolled at Colorado State University (Fort Collins, Colorado). Dogs were allocated to 1 of 3 groups (Group 1:5 mg CBD/kg BW daily, 5 mg/kg; Group 2:10 mg CBD/kg BW daily, 10 mg/kg; Group 3: vehicle [placebo] with 0 mg CBD/kg BW daily, 0 mg/kg) balanced by sex and body weight, and treatments were randomly assigned to each group (n = 6) using the random function in Microsoft Excel (Microsoft Corporation, Redmond, Washington). Before study start, it was determined that a minimum of 5 dogs per group would be required for >80% statistical power using ALP change data of dogs dosed with 4 mg/kg CBD for 28 days. ¹⁶ Participating dogs had an average age (±SD) of 2.3 ± 0.14 years (range, 2.1‐2.6 years), and average BW (±SD) of 9.5 ± 1.80 kg (range, 7.1‐12.8 kg) at study start. The study was approved by the Institutional Animal Care and Use Committee (IACUC) at Colorado State University (protocol number 2121).

Dogs belonging to the same treatment were housed either in pairs or trios in a single room. All dogs were offered adult dry dog food for maintenance (Hill's Science Diet Adult Chicken & Barley Recipe; Colgate‐Palmolive Company, New York, New York), with no other foods or treats. Food was pre‐weighed for each dog with the intention to maintain a healthy body fat index (BFI), ¹⁷ and amounts were adjusted every 2 weeks as deemed necessary. Feeding was conducted individually in separate kennels between 07:00 and 09:00 each day, and CBD or vehicle was dosed once daily within 30 minutes of feeding, using 3 mL syringes with 0.1 mL gradations. Treatment doses were adjusted once every 2 weeks according to the dogs' body weights. There were 2 instances during the 36‐week study (days 0‐2, days 126‐128) when dogs did not receive CBD for 1 day for the purpose of pharmacokinetic (PK) analyses. The last PK analysis (PK3) is presented in Figure 1, but was conducted after the last dose dogs received so that no other CBD doses were missed. Fresh water was always available. Dogs were socialized every day, including weekends and holidays, with veterinary students and other dogs within the same treatment group in a common playground area with artificial turf and natural sunlight. Upon study completion and verification of health data, all dogs were successfully adopted into private homes.

FIGURE 1.

Cannabidiol chronic dosing study timeline and monthly activities.

Before study initiation, dogs were adapted to their housing conditions for 3 weeks. Treatments were initiated on 15 November 2021 (day 0, baseline) and continued for the remainder of the study until 25 July 2022 (day 252 or week 36, Figure 1).

2.3. Assessment of dog health and sample handling

All study dogs had general physical examinations once every 4 weeks throughout the study period. During each physical examination, temperature, heart rate, respiratory rate, weight, and BFI were assessed. In addition, observations of skin, eyes, nose, mouth and teeth, heart and lungs, the abdomen, lymph nodes, mentation and personality, activity level, and vocalization were noted.

Blood was collected every 4 weeks to assess overall health and CBD plasma concentrations. Approximately 10 mL of blood was collected from the jugular vein pre‐prandially and divided into tubes in the following manner: 2 mL in purple‐top tube (BD Vacutainer K2 EDTA 3.6 mg; BD Company, Franklin Lakes, New Jersey) for plasma separation (cell blood count), 3 mL into 2 red‐top tubes (Monoject no additives; Covidien, Dublin, Ireland) for serum separation (chemistry and metabolomics), and 2 mL in green‐top tubes (BD Vacutainer sodium heparin 33 IU; BD Company, Franklin Lakes, New Jersey) for plasma (CBD). In sequence, dogs were fed, dosed with their respective oils, and blood again was collected 2 hours post‐prandially for bile acid and plasma CBD concentration measurements. Tubes were placed in ice and immediately centrifuged for 10 minutes at 2000 for serum and plasma separation (Avanti J‐15R centrifuge; Beckman Coulter Company, Pasadena, California). Next, both serum for metabolomics and plasma for CBD were divided into 2 cryotubes each and stored at −80°C for future work.

Adverse events were recorded. Fecal scores of 5, 6, or 7 on the Purina fecal scoring chart ¹⁸ also were recorded.

2.4. Statistical analysis

Parametric data were analyzed as repeated measures over time using the generalized linear mixed model (GLIMMIX) procedure in the statistical analysis software (SAS Institute v 9.4, Cary, North Carolina). Endpoints evaluated included indicators of liver function: ALP, alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma‐glutamyl transferase (GGT) activities, and bilirubin, fasted bile acid, post‐prandial bile acid, and albumin concentrations. The remaining CBC and blood biochemistry variables also were analyzed using JMP Pro v17 (SAS Institute Inc., Cary, North Carolina), but no significant changes were found. Means and SDs of all blood variables measured are reported in Table S1.

Data were assessed to meet model assumptions by plotting a studentized residuals panel using the option “plots = student panel” in GLIMMIX (SAS v 9.4). Residuals of ALP did not meet model assumptions, and data was natural logarithm transformed and then back transformed for reporting using the original scale. The baseline results were added as a covariate only for AST, because there were differences (P ≤ .05) among treatments at baseline. Fixed effects included time, treatment, and their interaction term, which was removed from the model when not significant (P > .05). Dog nested was added as the subject, and covariance structure was defined as unstructured (UN) for blood biochemistry variables under the random statement from GLIMMIX. Pairwise treatment comparisons were adjusted using Tukey‐Kramer post‐hoc tests to protect against Type I error. A P‐value ≤.05 was considered significant.

Fecal scores were analyzed as categorical (ordinal) data using a single chi‐squared test for all abnormal fecal scores (scores 5, 6, and 7 ¹⁸ ) across treatments (0, 5, and 10 mg/kg) recorded from week 0 to 36, using JMP Pro 17. Significance was found in the overall chi‐squared test (P < .05), and additional pairwise chi‐squared tests were performed between 0 and 5 mg/kg, 0 and 10 mg/kg, and 5 and 10 mg/kg to identify treatment differences.

3. RESULTS

3.1. Body weight and body condition change

Dogs maintained their overall health during the study, which was assessed by physical examinations, CBC, and blood biochemistry. At the beginning of the trial, 7 of the dogs had a BFI < 20 (Figure 2B), and BW started to increase significantly after week 12, mostly in the placebo group (P = .004; Figure 2A). This change also was captured by subjective evaluation of BFI (Figure 2B), where dogs receiving placebo scored a BFI of 30 more frequently as the study advanced.

FIGURE 2.

Body weight (means ± SD) (A) and body fat index (BFI) (B) of Beagles throughout the 36‐week dosing with 0, 5, and 10 mg/kg CBD.

Other continuous variables measured during physical examinations included temperature and heart rate, which were not different among treatments (P > .1). Variables measured during physical examinations were normal, except for minor observations from the examiners including mildly enlarged lymph nodes (6 instances in 0 mg/kg, 7 instances in 5 mg/kg, and 5 instances in 10 mg/kg), mild to moderate dental plaque, and some personality and mentation notes that were unrelated to treatment.

3.2. Blood variables

Most blood variables measured throughout the 36 weeks were within reference ranges and unaffected by treatment, except for ALP activity. Alkaline phosphatase activity tended to plateau after week 8, but large variations still occurred both within individual dog and week until the end of the trial.

Treatment alone had a significant effect (P < .0001) on ALP activity; wherein dogs dosed with the 5 and 10 mg/kg CBD oil had overall higher ALP activity than those given only the vehicle (Figure 3, Table 1). One outlier in the 10 mg/kg group had ALP activity between 935 and 1511 IU/L (7‐ to 10‐fold above the high end of the reference range) from week 8 to 36 but returned to a non‐clinically relevant activity (ALP = 336 UI/L) 2 weeks after the study ended. Alanine aminotransferase (ALT) remained within the reference range for all treatments (Figure 4; Table 1). A single dog had increased ALT activity at week 32 (the same dog with high ALP), that decreased to normal at week 36 (Figure 4). Aspartate aminotransferase activity was higher in the 5 mg/kg than in the 10 mg/kg group, the placebo group was similar to both treatment groups, but all dogs had activities within the reference range. Albumin concentration and GGT activity were not analyzed statistically because of the high number of repeated values in the dataset. Fasted and post‐prandial bile acids concentrations did not differ among treatment groups (P > .1). One outlier in the control group had increased post‐prandial bile acids concentrations (from 41 to 210 μmol/L) between weeks 12 and 36, but concentrations returned to within normal limits 2 weeks after the end of the trial.

FIGURE 3.

Box plots of ALP change over time of dogs supplemented daily with 10 mg/kg CBD (red), 5 mg/kg CBD (blue), and 0 mg/kg CBD (gray) for 36 weeks. Day 0 (baseline) was not included in the statistical analysis but was added in the graphic to show the starting point of the 3 treatment groups. Normal reference range for ALP is shown with dashed lines (15‐140 IU/L).

TABLE 1.

Least square means [95% confidence interval] of biochemistry variables of dogs chronically dosed with 0, 5, and 10 mg/kg CBD over 36 weeks.

Variable	0 mg/kg	5 mg/kg	10 mg/kg	Reference range	P
ALP, a IU/L	61.14B [40.1, 93.2]	284.5A [186.6, 433.9]	373.5A [244.9, 569.6]	15‐140	<.0001
ALT, IU/L	32.9 [26.3, 39.4]	29.1 [22.6, 35.7]	39.5 [33.0, 46.0]	10‐90	.09
AST, IU/L	26.3AB [23.4, 29.2]	30.8A [28.0, 33.6]	23.4B [20.3, 26.6]	15‐45	.01
Fasted bile acids, μmol/L	2.29 [1.38, 3.20]	1.86 [0.95, 2.78]	2.14 [1.21, 3.06]	0‐9	.76
Post‐prandial bile acids, a μmol/L	5.19 [2.79, 9.65]	7.67 [4.12, 14.26]	4.78 [2.57, 8.89]	10‐20	.48
BUN, mg/dL	11.9 [10.5, 13.3]	12.2 [10.8, 13.5]	12.7 [11.3, 14.1]	7‐30	.61

Note: Different superscript capital letters denounce significance between treatments within each variable.

^a ALP and post‐prandial bile acids were natural log transformed to meet model assumptions and converted back to original scale for reporting.

FIGURE 4.

Box plots of ALT change over time of dogs supplemented daily with 10 mg/kg CBD (red), 5 mg/kg CBD (blue), and 0 mg/kg CBD (gray) for 36 weeks. Day 0 (baseline) was not included in the statistical analysis but was added in the graphic to show the starting point of the 3 treatment groups. Normal reference range for ALT is shown with dashed lines (10‐90 IU/L).

3.3. Adverse events

The most prevalent AEs were gastrointestinal (GI) in nature, and all observed AEs were mild. ¹⁹ Differences in the frequency of soft feces and diarrhea were recorded among treatment groups (Figure 5). Dogs dosed with 10 mg/kg CBD oil had higher frequency (P < .01) of feces scored as 5, 6, and 7 (Purina fecal scoring chart) than dogs dosed with 5 mg/kg or placebo. The placebo and lowest dosing group were similar (P > .1). Dogs in the 10 mg/kg group also had a higher frequency of soft feces (13 episodes of fecal score 5, and 10 recordings of fecal score 6) than the 5 mg/kg group (4 episodes of fecal score 5) and placebo (12 episodes of fecal score 5, and 1 of fecal score 6) during the 3‐week baseline. Specifically, a single dog in the 10 mg/kg CBD oil treatment group had more frequent soft feces during baseline and the study period. Considering that this dog might have biased the data, we reanalyzed the chi‐squared tests without data of this individual dog, and the conclusion did not change. Thus, it was determined that the most concentrated broad‐spectrum hemp extract caused softer feces than the other oils.

FIGURE 5.

Counts and percentages of abnormal fecal scores (soft to liquid) of dogs given 3 doses of CBD (0, 5, and 10 mg/kg) during 36 weeks (does not include baseline). P(chi‐square) = .0001. Individual dots represent a fecal score and colors denote fecal score (5‐green, 6‐orange, and 7‐purple).

Adverse events that occurred more than once during the study period in all treatments included soft feces or diarrhea (Figure 5), with straining or tenesmus noted on several occasions and vomiting, including 5 episodes in the placebo, 6 episodes in the 5 mg/kg group, and 8 episodes (plus 1 episode of regurgitation) in the 10 mg/kg group. Emesis was unlikely related to treatment, because it was present in all groups at low frequency. Other less prevalent AEs that were recorded and considered unrelated to treatment included 8 mild events of dermatological nature (4 in the 0 mg/kg group, 4 in the 5 mg/kg group), and 1 case of foreign body ingestion in the 5 mg/kg group.

4. DISCUSSION

Many veterinary CBD products are available in the market, ²⁰ , ²¹ but evidence of their sustained tolerability long‐term in the scientific literature is scarce. Thus, our goal was to determine the tolerability of long‐term CBD administration, because it may be used for chronic conditions such as anxiety, osteoarthritis, and epilepsy. Because there is no conclusive evidence of the dosing and frequency of CBD supplementation to treat various diseases in dogs, ²² , ²³ , ²⁴ , ²⁵ 5 and 10 mg/kg BW once daily doses were chosen based on what has been previously tested in dogs, ²⁵ , ²⁶ , ²⁷ as well as on what has been found in humans and rodents to have anxiolytic and anticonvulsive effects. ⁵ , ²⁸

Cannabidiol‐based veterinary products usually are derived from hemp (Cannabis sativa), which contains <0.3% psychoactive THC by dry weight. ²⁹ Hemp can be processed into full spectrum, distillate (or broad spectrum), and CBD isolate. Full spectrum refers to minimally processed hemp with 10% to 25% CBD along with >100 PCs and other plant compounds (terpenes, flavonoids, fatty acids), whereas distillate is created after the complete removal of THC, and CBD isolate is the purified powder form of CBD. ²⁹ A distillate hemp extract suspended in MCT oil was used in our study because it includes other plant compounds that may enhance the therapeutic effect of CBD, such as terpenes. Terpenes are aromatic compounds responsible for the strong hemp taste and smell, and are thought to act synergistically with CBD, potentiating its effect in a phenomenon called entourage effect. ³⁰ , ³¹ This effect might also help explain why botanical extracts are often more efficacious than their isolated drug forms. ³¹

Cases of marijuana intoxication in the United States and Canada have been caused mostly by ingestion of THC‐containing products formulated for humans, both as edibles and dried cannabis, and most prevalent clinical signs include urinary incontinence, disorientation, ataxia, lethargy, hyperesthesia, and bradycardia. ³² A safety study testing escalating doses of CBD, THC : CBD, and THC oil in dogs found that the CBD oil at a single PO dose of 64.7 mg/kg CBD and 2.4 mg/kg THC (over 6 times higher than the highest CBD dose in our study) induced AEs that included nausea, emesis, diarrhea, lethargy, hyperesthesia, muscle tremor, and ataxia. ¹⁴ Moderate and severe AEs only occurred in dogs that received THC and CBD : THC oils. ¹⁴ Studies testing the safety of short‐term CBD use in dogs dosed PO with broad‐spectrum hemp extracts at dosages ranging from 2 to 20 CBD mg/kg/day for 2 to 12 weeks reported either no or only mild AEs. ¹⁶ , ²² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ In our study, the only AE noted in dogs supplemented with the highest dose was the higher frequency of soft feces or diarrhea with tenesmus and straining. In another study, diarrhea was a reported AE in dogs receiving oil‐based CBD extract, but the study lacked a negative control group, ³³ , ³⁶ and thus no clear conclusions could be made. In our study, the 2 treatment oils were developed to deliver their respective CBD doses in 1 ± 0.2 mL, so that the volume of oil received would not be a confounding factor. Also, we confirmed using statistics that the dog with propensity to produce soft feces in the 10 mg/kg group did not bias the results. Hence, the 100 mg/mL oil might have caused some GI upset because of its higher concentration of hemp extract.

Neurological examinations were not conducted in our study, and no behavioral changes were noted either on the general physical examinations or during socialization. The dog population in our study had daily interactions with the staff and students which likely contributed to their socialization, but this was not quantified. The controlled feeding regimen with a high‐quality Association of American Feed Control Officials (AAFCO) compliant dry adult dog food for maintenance also improved their overall health, as monitored by BW, BFI, and blood variables. In a similar long‐term CBD dosing study, ¹⁵ assessing daily life quality scores across the domains “happiness,” “energetic,” “mobile,” “relaxed,” and “sociable,” reported no differences between control and treatment groups. In our study, observations made by the examiners were mostly on the dogs' personalities rather than treatment‐related changes.

Dogs in our study were fed their daily amounts with the intention to maintain a healthy BW. Because some dogs were underweight at study start, a positive weight change occurred throughout the study, and a few dogs became slightly overweight by the end of the trial. The BFI scoring system also indicated that there were more overweight dogs in the placebo group. Although subjective scales must be used with caution because they may not reflect the true body fat mass in the dog, ³⁸ the placebo group had a higher overall mean BW, despite not being significantly different from the other treatment groups, which corroborated the subjective data. Thus, CBD itself likely did not cause weight gain. The primary influence on weight gain was the daily food amount offered with the intention of targeting a healthy condition, followed by the offering of kibbles as reward during daily enrichment, which led some dogs to become overweight.

The most comparable CBD long‐term tolerability study evaluated the effects of administering a CBD distillate at 4 mg/kg once daily for 6 months to 48 dogs (including Labrador retrievers, beagles, and Norfolk terriers) balanced by age, sex, and breed. ¹⁵ This study also found an increase in ALP activity that plateaued at week 4. ¹⁵ In our study, we found that ALP activity increased until the second month of the study, and monthly variations with a significant time effect were observed. The average serum ALP activity in the other study ¹⁵ remained just above the upper reference limit, whereas the dogs in our study receiving 5 mg/kg CBD (1 mg/kg additional per day) had a mean ALP activity of almost twice the upper reference range over the 9 months. It may be that the different CBD forms influenced ALP activity. In another recent study where CBD was supplemented at 5 mg/kg once daily for 28 days, ³⁹ mean ALP activity ranged between 100 and 150 UI/L by the last day of the trial, whereas beagles in our study receiving the same CBD dosage had a mean of nearly 200 UI/L after the first month. In the previous study, various mixed‐breed dogs were used, ³⁹ and it is expected that serum ALP activity will vary according to dog breed. ⁴⁰ , ⁴¹ Shorter term studies administering CBD to dogs at dosages 4 to 10 mg/kg/day reported an increase in serum ALP activity usually by the fourth week, with substantial intraindividual variation. ²⁵ , ³³ , ³⁵ , ³⁶ , ³⁷ Although a combination of CBD and cannabidiolic acid (CBDA) at 2 mg/kg twice daily for 12 weeks presented as soft chews increased ALP activity, activity did not exceed the upper reference limit. ³⁶ Therefore, ALP activity may be affected by CBD frequency of administration, the vehicle, and dog breed or size. Our study had large intraindividual variations of ALP activity.

Alkaline phosphatase is a cell membrane‐bound glycoprotein that carries zinc in its interior and catalyzes the hydrolysis of phosphate monoesters at alkaline pH, but its specific physiological functions are still unknown. ⁴² Four ALP isozymes are described in domestic animals: ALP originating from bone, intestine, liver, as well as corticosteroid‐induced ALP. ⁴³ , ⁴⁴ Because ALP is not liver‐specific, its increase alone does not indicate liver damage, but monitoring of hepatic function is important because CBD was found to both inhibit and activate cytochrome P450 enzymes in the liver of humans. ⁴⁵ Humans supplemented at a high dose of CBD (1500 mg/day) for approximately 3.5 weeks experienced increases in the hepatic marker ALT, and 31% had increases 5 times the upper reference limit that met international consensus criteria for drug‐induced hepatic damage. ⁴⁶ It is possible that similar dosages of CBD in dogs could induce hepatic damage, but evidence of damage was not seen in our study at 5 and 10 mg/kg BW/day CBD. Combination of cannabinoid types and interspecies differences may need to be taken into consideration for companion animals, because cats given 2 mg/kg of equal parts CBD and CBDA PO twice daily experienced AEs including frequent licking, headshaking, and 1 cat had increased ALT activity after 4 weeks. ³⁶ In our study, the lower CBD dose was better tolerated with respect to increases in liver enzyme activity than the higher dose, demonstrated by the lack of ALT increase in the lower dose group. Dogs supplemented with the 10 mg/kg CBD dose had high ALP activity, but not significantly as compared with the 5 mg/kg dosing group. Only 1 male dog at month 8 had ALT activity (97 IU/L) above the upper reference limit (90 IU/L), which returned to normal levels 2 weeks post‐study. Liver biomarkers including pre‐ and post‐prandial bile acids, AST, GGT, and bilirubin were measured, and no dog in groups receiving CBD had increases noted during the 9 months. Likewise, other biomarkers determined by monthly CBC and blood biochemistry also did not indicate any treatment effect.

5. CONCLUSION

In the 36 weeks of chronic CBD administration, dogs supplemented with 5 mg/kg CBD once daily had a similar frequency of soft feces and diarrhea as compared with the control group, and an overall increase in serum ALP activity without concomitant changes in other liver function tests. Conversely, dogs receiving 10 mg/kg CBD had a higher frequency of abnormal fecal scores, mean ALP activity was on average greater than in dogs in the lower dosing group (but not significantly). Our data indicate that CBD chronically administered at 5 mg/kg once daily is well tolerated by healthy dogs, and a higher dosage should be used with caution. Monitoring of liver function is recommended to assure that no other abnormalities occur, especially when dogs have concomitant conditions or receive other medications that also are metabolized by the liver. If liver function tests become abnormal, CBD should either be decreased or discontinued. Future work should focus on the tolerability of CBD offered long‐term at alternate time intervals, using different formulations (cannabinoid profile and vehicle), and in various dog breeds, ages, and sizes.

CONFLICT OF INTEREST DECLARATION

Kim Wilson is employed by Colgate‐Palmolive Company, who funded this work. Stephanie McGrath consults for a cannabidiol company. Kim Wilson is not affiliated with the Global Cannabis Partnership or any other cannabis associations. Kim Wilson and Stephanie McGrath were blinded to treatments during data collection and analysis. Isabella Corsato Alvarenga has no conflict of interest.

OFF‐LABEL ANTIMICROBIAL DECLARATION

Authors declare no off‐label use of antimicrobials.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) OR OTHER APPROVAL DECLARATION

Approved by the IACUC at Colorado State University (protocol number 2121).

HUMAN ETHICS APPROVAL DECLARATION

Authors declare human ethics approval was not needed for this study.

Supporting information

Table S1. Mean ± SD of blood parameters of dogs supplemented once a day with 0, 5, or 10 mg CBD/kg BW for 36 weeks.

Corsato Alvarenga I, Wilson KM, McGrath S. Tolerability of long‐term cannabidiol supplementation to healthy adult dogs. J Vet Intern Med. 2024;38(1):326‐335. doi: 10.1111/jvim.16949