Is there evidence of vitamin B1 (thiamine) deficiency associated with glucagon-like peptide‑1 (GLP‑1) receptor agonist use?

Vitamin B1 (Thiamine) Deficiency with GLP-1 Receptor Agonist Use

There is emerging evidence that GLP-1 receptor agonists can precipitate thiamine (vitamin B1) deficiency, particularly in patients experiencing prolonged gastrointestinal symptoms, rapid weight loss, or restrictive dietary intake. While this is not listed as a recognized adverse effect in current clinical guidelines 1, recent case reports demonstrate serious and potentially irreversible neurological complications including Wernicke's encephalopathy and Korsakoff syndrome in patients using these medications 2, 3, 4.

Evidence of Thiamine Deficiency Risk

The mechanism appears multifactorial:

• Reduced nutritional intake: GLP-1 receptor agonists cause marked appetite suppression and delayed gastric emptying, leading to substantially decreased food consumption 1
• Persistent gastrointestinal symptoms: Nausea, vomiting, and diarrhea—the most common adverse effects—can persist beyond the initial treatment phase and impair nutrient absorption 1
• Rapid weight loss: Users without diabetes experience 6.1-17.4% weight loss, with some achieving 14.9-20.9% reduction 1, creating increased metabolic demand for thiamine

Clinical Case Evidence

A systematic review identified six cases of Wernicke's encephalopathy following semaglutide treatment for obesity 4. All patients experienced:

• Prolonged gastrointestinal symptoms before neurological deterioration
• Substantial weight loss
• Common manifestations of altered mental status and oculomotor abnormalities
• Poor outcomes including progression to Korsakoff syndrome or death

One case report documented severe perioperative lactic acidosis and hyperglycemia in a patient on semaglutide with restrictive dieting, which resolved rapidly with intravenous thiamine 2. Another case described Korsakoff syndrome development within weeks of tirzepatide initiation in a patient with chronic alcohol use, despite unremarkable initial brain MRI 3.

Broader Micronutrient Deficiency Pattern

A narrative review of 480,825 adults found 5:

• Vitamin D deficiency: 7.5% at 6 months, 13.6% at 12 months
• Iron depletion: 26-30% lower ferritin levels compared to SGLT2 inhibitor users
• Inadequate intake: >60% consumed below requirements for calcium and iron; vitamin D intake averaged only 20% of recommendations
• Progressive deficiencies: Thiamine and cobalamin deficits increased over time

High-Risk Patient Profiles

Immediate thiamine supplementation should be considered for patients with:

• Chronic alcohol use disorder (even if abstinent)
• History of bariatric surgery, especially if GLP-1 therapy initiated within months of surgery 6
• Persistent gastrointestinal symptoms (>2-4 weeks of nausea/vomiting)
• Rapid weight loss exceeding 10% body weight in 3 months
• Restrictive dietary patterns or very low calorie intake
• Any neurological symptoms (confusion, ataxia, vision changes, weakness)

One case demonstrated severe lumbosacral polyradiculopathy requiring inpatient rehabilitation in a patient on semaglutide post-bariatric surgery, which improved with thiamine and vitamin E supplementation after medication discontinuation 6.

Clinical Pitfalls

Critical warning: Wernicke's encephalopathy can present with normal initial brain MRI 3. Do not rely on imaging alone. The classic triad (confusion, ataxia, ophthalmoplegia) is present in only 10-16% of cases in general populations.

Do not wait for laboratory confirmation before treating suspected thiamine deficiency—irreversible Korsakoff syndrome can develop within days. Administer parenteral thiamine (500 mg IV three times daily for 3-5 days) immediately if clinical suspicion exists 4.

Practical Management Algorithm

1. At GLP-1 initiation: Screen for alcohol use, prior bariatric surgery, eating disorders, and baseline nutritional status
2. During titration: Monitor for persistent GI symptoms beyond 4 weeks; if present, consider empiric thiamine supplementation (100 mg daily oral minimum)
3. With rapid weight loss: If >10% body weight loss in 3 months, obtain thiamine level and supplement regardless of result
4. Any neurological symptoms: Immediate parenteral thiamine 500 mg IV TID, do not delay for testing
5. Post-bariatric surgery: Avoid initiating GLP-1 therapy within 6-12 months of surgery; if necessary, mandatory thiamine supplementation 6

Guideline Gap

Current guidelines list only gastrointestinal symptoms, gallbladder disorders, pancreatitis, and cardiac arrhythmias as adverse effects 1. Thiamine deficiency is not mentioned in any major diabetes or obesity treatment guidelines 7, 8, 9, representing a significant knowledge gap as GLP-1 prescriptions reach millions of patients 10.

The evidence base consists primarily of case reports and observational data 2, 3, 4, 5, 6, but the severity of outcomes (death, permanent cognitive impairment) and preventability with simple supplementation warrant heightened clinical vigilance. Integrated nutritional assessment is recognized as important by obesity societies but rarely provided in practice 10.

Bottom line: While not yet in formal guidelines, clinicians should maintain a low threshold for thiamine supplementation in GLP-1 users, particularly those with risk factors, and should educate patients about maintaining adequate protein and micronutrient intake despite appetite suppression.