Vitamin B12 toxicity secondary to post-bariatric supplementation: a case report on presentation and literature review

Abstract

Introduction:

Bariatric surgery leads to nutritional alterations, particularly affecting micronutrient levels. Vitamin B12 plays essential roles in neurological function and hematopoiesis. Postoperative changes in gastrointestinal anatomy disrupt B12 absorption, often necessitating supplementation. However, excessive supplementation may result in supraphysiological levels, with potential adverse effects. This report explores such a scenario to raise awareness about the need for balanced vitamin B12 management.

Case Presentation:

A 43-year-old male with obstructive sleep apnea underwent laparoscopic sleeve gastrectomy with an initial body mass index (BMI) of 38. Postoperative recovery was uneventful. He was prescribed multivitamins, vitamin D, and vitamin B complex. By the third month, he achieved a BMI of 25 but reported peripheral extremity weakness and numbness. Lab tests showed a vitamin B12 level of 1300 pg/mL (picograms per milliliter). Supplementation was halted, and after 1 month, symptoms resolved, and levels normalized to 617 pg/mL. The patient resumed normal activity and was advised regular B12 monitoring.

Discussion:

Vitamin B12 absorption is dependent on intrinsic factor and intact gastrointestinal anatomy. Bariatric surgeries alter this physiology, increasing the risk for both deficiency and excessive supplementation. Current literature highlights that while Roux-en-Y gastric bypass patients often develop deficiencies, Vertical sleeve gastrectomy patients may inadvertently accumulate B12 due to altered absorption patterns. Supraphysiological B12 levels may lead to oxidative stress and obscure other deficiencies. This case supports the necessity of tailored supplementation regimens and ongoing monitoring.

Conclusion:

This report underscores the importance of vigilant vitamin B12 monitoring in post-bariatric patients. Individualized supplementation plans and routine biochemical assessments are crucial to prevent both deficiency and toxicity. Further studies should investigate the long-term effects of high B12 levels in this population.

Introduction

Bariatric surgery causes nutritional deficits that may exacerbate underlying micronutrient deficiencies^[1^]. Vitamin B12 plays a crucial role in neurological function, red blood cell formation, and DNA synthesis. Its absorption relies on intrinsic factor produced in the stomach, and alterations in the gastrointestinal anatomy due to bariatric procedures can disrupt this process, leading to deficiency or, in some cases, unintended excess due to supplementation. Understanding the metabolic changes affecting vitamin B12 levels in post-bariatric surgery patients is essential to prevent both deficiency and toxicity and to ensure optimal patient outcomes. Vitamin B12 deficiency is a well-documented complication of bariatric surgery, primarily due to reduced dietary intake and malabsorption^[2,3^]. However, excessive supplementation can also pose risks, leading to supraphysiological B12 levels with potential adverse effects^[4^].

HIGHLIGHTS

• Excessive vitamin B12 supplementation post-bariatric surgery can result in supraphysiological levels and neurological symptoms.

• A 43-year-old male developed peripheral neuropathy due to high serum B12 levels 3 months after sleeve gastrectomy.

• Symptom resolution occurred within 1 month after discontinuation of supplementation and normalization of vitamin B12 levels.

• This case emphasizes the need for individualized supplementation protocols and routine biochemical monitoring.

• Balanced management of vitamin B12 post-bariatric surgery is critical to avoid both deficiency and toxicity.

Current standards of care for post-bariatric patients emphasize routine micronutrient surveillance and individualized supplementation strategies. Guidelines by the British Obesity and Metabolic Surgery Society (BOMSS) and American Society for Metabolic and Bariatric Surgery (ASMBS) recommend periodic biochemical monitoring, especially for vitamin B12, due to the high prevalence of deficiency and the emerging recognition of toxicity risks^[3,5^]. This case is significant because it documents a rare but clinically relevant presentation of vitamin B12 toxicity following routine supplementation in a post-sleeve gastrectomy patient. While deficiency is widely acknowledged, toxicity is underreported, particularly in the Middle East population. This case underscores the need for vigilant biochemical monitoring even in asymptomatic patients, and it adds to the limited literature on post-bariatric B12 excess.

This case report has been reported in line with the SCARE checklist 2025^[6^].

Case presentation

A 43-year-old male presented to the outpatient bariatric clinic at a private tertiary hospital for routine follow-up after undergoing a laparoscopic sleeve gastrectomy (LSG) for class II obesity [body mass index (BMI) 38 kg/m³] in February 2024. The chronological sequence of postoperative clinical events is summarized in Table 1. His past medical history included obstructive sleep apnea (OSA), effectively managed with continuous positive airway pressure (CPAP). He had no prior surgical interventions, hospitalizations, or significant past medical history. He was not taking any chronic medications aside from prescribed postoperative supplements. There were no known drug or food allergies.

Table 1.

Timeline of clinical events.

Date	Time	Event
10/02/2024	08:00	Patient underwent laparoscopic sleeve gastrectomy
24/02/2024	10:30	2-week follow-up; prescribed multivitamins, vitamin D, and B complex
10/03/2024	11:00	1-month follow-up; patient progressing well
10/05/2024	09:00	3-month follow-up; onset of peripheral neuropathy symptoms
15/05/2024	08:45	Labs reveal supraphysiological vitamin B12 level (1300 pg/mL)
16/05/2024	08:00	Vitamin B12 supplementation discontinued
15/06/2024	09:00	Symptoms resolved; serum B12 normalized (617 pg/mL)
01/07/2024	09:00	Returned to baseline function; continued under routine follow-up

He was a non-smoker, denied alcohol or recreational drug use, lived independently in private accommodation, and drove himself to clinic appointments. He had no history of inherited diseases in first-degree relatives.

Preoperative optimization involved CPAP compliance for OSA, standard preoperative counseling, fasting 8 hours before surgery, and psychological readiness assessment. All preoperative laboratory investigations, including serum vitamin levels and serum vitamin B12 concentrations, were within normal limits. The LSG was performed by a senior bariatric surgery consultant under general anesthesia, using standard laparoscopic technique and sterile precautions. No intraoperative complications occurred. Postoperatively, the patient received analgesia, antiemetics, and venous thromboembolism prophylaxis. He was discharged on postoperative day 1 with instructions on wound care, activity, and nutrition. There were no deviations from the initial surgical plan.

Our institution follows a structured postoperative surveillance pathway for bariatric patients, emphasizing nutritional monitoring, micronutrient supplementation, and dietitian-led dietary progression. Follow-up assessments are routinely scheduled at 2 weeks, 1 month, 3 months, 6 months, and 12 months, and subsequently every 6–12 months. Each visit includes measurement of weight, BMI, and dietary intake, as well as a review of adherence to prescribed supplements. A registered clinical dietitian provides individualized nutritional counseling, and micronutrient profiles (including vitamin B12, vitamin D, folate, iron studies, calcium, parathyroid hormone, zinc, and copper) are obtained periodically in accordance with clinical indications.

Supplementation protocols differ based on procedure type due to varying risks of malabsorption. Patients undergoing LSG receive a standard regimen consisting of daily multivitamin and vitamin B complex for 6 months, ursodeoxycholic acid 250 mg once daily for 6 months in those without prior cholecystectomy, and vitamin D3 50,000 IU once weekly for 3 months. Routine calcium or iron supplementation is not required after sleeve gastrectomy unless a deficiency or risk factor is identified, as gastrointestinal continuity remains intact.

In contrast, patients undergoing Roux-en-Y gastric bypass (RYGB) receive additional prophylactic supplementation to mitigate malabsorptive consequences of the bypassed duodenum and proximal jejunum. This includes calcium carbonate 600 mg twice daily for 3 months and ferrous fumarate 200 mg twice daily for 3 months, administered alongside the core supplements used for sleeve gastrectomy patients. This tailored approach aims to reduce the incidence of micronutrient deficiencies during the rapid weight-loss phase and throughout long-term postoperative recovery.

At the 2-week follow-up (Table 1), the patient was prescribed a routine postoperative regimen that included multivitamin once daily (standard bariatric formulation containing: vitamin A 5000 IU, vitamin D3 800 IU, vitamin E 15 mg, vitamin K 120 mcg, folic acid 400 mcg, iron 45 mg, zinc 11 mg, copper 2 mg), vitamin D3 2000 IU once daily, and a vitamin B complex [Thiamine (vitamin B1) 50 mg, riboflavin (vitamin B2) 20 mg, niacin (vitamin B3) 50 mg, pyridoxine (vitamin B6) 50 mg, cyanocobalamin (vitamin B12) 500 mcg, biotin 300 mcg, pantothenic acid 50 mg]. At the 1-month follow-up, he demonstrated significant weight loss progress and reported no complaints.

The patient’s weight was monitored at each scheduled postoperative visit, with documented progressive weight loss consistent with expected post–sleeve gastrectomy outcomes. His dietary progression from liquids to pureed textures, soft diet, and eventually regular bariatric portions was closely supervised by a licensed clinical dietitian. Ongoing nutritional counseling ensured adherence to protein targets, hydration goals, and micronutrient intake.

At the 3-month follow-up in May 2024 (Table 1), the patient reported bilateral upper and lower limb numbness and mild weakness, which interfered with both daily activities and work. There was no history of trauma, blurred vision, chest pain, or abdominal discomfort. Review of systems was otherwise unremarkable. On clinical examination, the patient had reduced sensation to light touch and vibration in the distal upper and lower limbs, with subjective weakness.

Laboratory investigations revealed a serum vitamin B12 level of 1300 pg/mL (normal range: 200–900 pg/mL). No additional blood or imaging investigations were performed, as clinical findings and supplement history suggested B12-related neuropathy. Differential diagnoses considered included neuropathy, vitamin B12 deficiency, and other micronutrient imbalances. Vitamin B12 deficiency was initially suspected, given its higher prevalence in bariatric patients, but was excluded by elevated levels. The repeat test confirmed the elevated vitamin B12 level, supporting a true supraphysiological result rather than error. There were no access, financial, or cultural challenges during diagnosis or follow-up.

Vitamin B complex supplementation was discontinued. At 1-month follow-up in June 2024 (Table 1), symptoms had fully resolved and serum B12 levels normalized to 617 pg/mL. The patient resumed full daily and occupational activities. He was advised to continue follow-up every 3 months with serum B12 monitoring, as part of a long-term post-bariatric care plan. No complications or adverse outcomes were noted, and adherence to follow-up and clinical guidance was excellent.

Discussion

Vitamin B12 absorption begins in the stomach, where the acidic environment facilitates its degradation and binding to intrinsic factor^[7^]. The complex is then absorbed in the ileum, with initial processing occurring in the duodenum^[8^]. Alterations in the gastrointestinal tract following bariatric procedures, such as duodenal switch or gastric bypass, can significantly reduce vitamin B12 absorption^[9^]. Studies suggest that vitamin B12 deficiency is more common after RYGB compared to vertical sleeve gastrectomy (VSG)^[10^]. However, recent findings indicate that patients undergoing VSG may also be prone to developing supraphysiological B12 levels due to altered gastric and intestinal structures^[11^].

In this case, the patient developed peripheral neuropathic symptoms 3 months after VSG, which were attributed to elevated vitamin B12 levels. Discontinuation of supplementation led to symptom resolution and normalization of B12 levels. This supports the rationale that excessive supplementation, even in non-malabsorptive procedures like VSG, can lead to clinically relevant toxicity.

Vitamin B12 supplementation recommendations vary based on the mode of administration and type of bariatric procedure^[12^]. Generally, either a monthly intramuscular injection of 1000 μg or a daily oral dose of 350–1000 μg is advised^[5^]. While most patients tolerate high B12 levels without adverse effects, a subset may experience complications such as oxidative stress and metabolic disruptions^[13^]. Elevated B12 levels can also obscure or delay the detection of other deficiencies, such as folate^[3^].

Relevant case reports have highlighted similar risks associated with excessive vitamin B12 intake^[2^]. Supraphysiological B12 levels have been linked to oxidative stress, altered DNA methylation, and disruption of metabolic pathways – potentially exacerbating conditions like diabetes or increasing susceptibility to malignancy. For example, one report described a middle-aged woman who developed bilateral visual impairment resembling toxic optic neuropathy following high-dose B12 intake post-surgery^[13^]. These findings underscore the importance of balanced supplementation to prevent both deficiency and toxicity^[6^].

Recent international guidelines on post-bariatric surgery follow-up including ASMBS and BOMSS recommendations emphasize structured and lifelong monitoring of micronutrients, with routine assessment of vitamin B12, vitamin D, folate, iron studies, calcium, parathyroid hormone, zinc, and copper at defined postoperative intervals^[3,4^]. These guidelines also highlight the need for procedure-specific monitoring, with more intensive testing recommended for malabsorptive operations such as RYGB compared to sleeve gastrectomy.

In alignment with these guidelines, our institutional protocol incorporates scheduled micronutrient assessment at 2 weeks, 1 month, 3 months, 6 months, 12 months, and every 6–12 months thereafter. All bariatric patients receive standardized supplementation, with additional iron and calcium provided specifically to RYGB patients due to their higher malabsorption risk. This structured approach enabled early recognition of elevated vitamin B12 in our patient and allowed for timely intervention.

Monitoring excess vitamin B12 levels requires serial serum measurements, correlation with clinical symptoms, and evaluation for potential non-supplement-related etiologies. Persistent supraphysiological levels warrant consideration of hepatic dysfunction, hematologic malignancies, renal failure, or other systemic illnesses. Although supplementation was clearly responsible in this case, clinicians should remain vigilant about these alternative causes when serum B12 values are unexpectedly high.

Other recognized causes of elevated B12 include chronic liver disease, myeloproliferative disorders such as polycythemia vera or chronic myelogenous leukemia, solid organ malignancies, and renal dysfunction. Acknowledging these possibilities provides a more complete differential diagnosis and ensures appropriate evaluation when findings are discordant with expected postoperative physiology. In our patient, normal preoperative laboratory results, the absence of systemic symptoms, and the rapid decline in serum B12 after discontinuation of supplements effectively ruled out these conditions.

Moving forward, this case emphasizes the need for routine and individualized micronutrient monitoring, even in patients undergoing restrictive bariatric procedures. Incorporating B12 level checks into standard follow-up protocols may prevent iatrogenic complications.

Clinicians should maintain a high index of suspicion for supplement-related complications, even when patients appear to be adherent to postoperative guidelines. Regular B12 monitoring, personalized supplementation plans, and prompt assessment of new-onset neuropathic symptoms are critical. Future management protocols should also consider early education about the risks of over-supplementation.

Strengths of this report include the clear temporal relationship between supplementation and symptom onset, the confirmation of elevated B12 using repeat laboratory analysis, and the rapid symptom resolution following targeted treatment. Limitations include the single-patient design and limited duration of follow-up, which constrain the generalizability of the findings and prevent broader conclusions about long-term risk.

Conclusion

This case highlights the importance of routine and individualized monitoring of vitamin B12 levels in post-bariatric surgery patients. Although deficiency is more commonly anticipated, excessive supplementation can also lead to clinically significant yet reversible neurological symptoms, even in restrictive procedures such as sleeve gastrectomy. Early identification of abnormal vitamin levels, coupled with timely adjustment of supplementation, is essential to prevent iatrogenic complications. Incorporating structured micronutrient surveillance into standard postoperative care pathways may help optimize outcomes and reduce both deficiency- and toxicity-related risks. Further research is needed to better define the prevalence, mechanisms, and long-term implications of vitamin B12 excess following bariatric surgery.

Ethical approval

Institutional review board waiver obtained from the institution, as this is a single anonymized case report without experimental intervention.

Consent

Written informed consent was obtained from the patient for both the surgical intervention and the publication of this case report. Documentation of the consent is available upon request.

Sources of funding

This research received no external funding.

Author contributions

Y.A.: Writing, Editing, Manuscript finalization. N.A.: Original drafting, Design and implementation of the research, Data collection. A.A.: Writing, Editing, Data collection. S.A.: Data collection, Editing. A.A.: Treating physician, Critical revision of the article, Manuscript drafting. All authors contributed to the conception, drafting, revision, and final approval of the manuscript. All authors meet the ICMJE authorship criteria.

Conflicts of interest disclosure

The authors declare no conflicts of interest.

Research registration unique identifying number (UIN)

Not applicable.

Guarantor

Amer Alshahrani.

Provenance and peer review

Not commissioned, externally peer-reviewed.

Data availability statement

Data sharing is not applicable to this article as no datasets were generated or analysed for this case report.

Presentation history

This case has not been presented at any conference or regional meeting.