Medical Disclaimer: This article is for educational purposes only and is not intended as medical advice. If you are taking Ozempic or any medication, consult your healthcare provider before making any changes to your diet, supplementation, or medication regimen. Do not stop taking Ozempic without medical supervision. Seek immediate emergency care if you experience severe neurological symptoms such as confusion, memory loss, loss of coordination, or vision changes, which may indicate Wernicke-Korsakoff syndrome.
Why This Matters: The B1-Ozempic Connection
If you’re taking Ozempic or other GLP-1 receptor agonists for diabetes or weight management, you’ve likely experienced significant appetite suppression and weight loss—key reasons why millions of people use these medications. However, a growing body of clinical research is sounding an alarm about a rare but serious nutritional side effect: vitamin B1 (thiamine) deficiency. Most documented cases were reported in 2023-2024, and research analyzing them continues to be published. Left untreated, B1 deficiency can develop into Wernicke-Korsakoff syndrome, a potentially irreversible neurological condition affecting memory, coordination, and vision. This article explains the connection, who is at risk, and what you can do to protect yourself while using Ozempic.
What Is Ozempic and Who Uses It?
Ozempic is a brand name for semaglutide, a medication originally developed to help people with type 2 diabetes manage blood sugar. More recently, higher-dose versions (like Wegovy) have been approved for chronic weight management. GLP-1 receptor agonists like semaglutide work by mimicking a natural hormone that signals fullness to the brain, dramatically reducing appetite and slowing gastric emptying—the rate at which food moves through your stomach.
The medication has gained enormous popularity; millions of people now use semaglutide for weight loss, sometimes prescribed off-label by physicians. While generally considered safe for blood sugar and heart health, the appetite suppression can be so profound that users report little interest in food for weeks at a time. This extreme reduction in food intake, combined with gastrointestinal side effects like nausea, diarrhea, and vomiting, creates conditions for nutritional deficiencies—including vitamin B1 deficiency.
What Is Vitamin B1 (Thiamine)?
Thiamine, also known as vitamin B1, is an essential water-soluble vitamin your body cannot produce on its own. You must obtain it from food or supplements. B1 plays critical roles in:
- Energy metabolism: Converting carbohydrates into the energy your cells use
- Nerve function: Supporting healthy neurological signaling and myelin (nerve insulation) formation
- Brain health: Essential for memory, learning, and cognitive function
- Heart function: Necessary for proper cardiac muscle contraction
Daily requirements are modest—adult men need about 1.2 mg, and adult women need about 1.1 mg—but that small amount is absolutely vital. Rich dietary sources include pork, chicken, fish, whole grains, fortified cereals, nuts, and legumes. However, if appetite suppression or gastrointestinal dysfunction prevents you from eating normally, B1 intake can plummet, even if you’re aware you need it.
How Ozempic May Lead to B1 Deficiency: The Science
The mechanism connecting GLP-1 agonists to B1 deficiency involves several overlapping pathways:
1. Severe appetite suppression: Semaglutide works by signaling satiety to the brain. Many users report feeling so full from small amounts of food that eating becomes a deliberate effort. If you typically eat a 2,000-calorie diet and suddenly eat 800 calories or fewer, your micronutrient intake—including B1—drops proportionally.
2. Gastrointestinal side effects: Common adverse effects include delayed gastric emptying, nausea, vomiting, and variable diarrhea or constipation. These symptoms further reduce food intake and impair nutrient absorption in the gut.
3. Reduced stomach acid and enzyme production: Slowed gastric function can impair the breakdown and absorption of nutrients, including thiamine.
Recent research confirms this is a real, if uncommon, phenomenon. A 2026 case report in Case Reports in Neurology described a patient who developed severe sensorimotor neuropathy—weakness, numbness, and cranial nerve deficits—after losing 120 pounds while on semaglutide with persistent GI intolerance; laboratory testing confirmed deficiencies in thiamine, folate, and B12, and the patient improved significantly within five days of vitamin supplementation (Cohan et al., 2026).
Separately, a pharmacovigilance study analyzing the FDA Adverse Event Reporting System (FAERS) identified 15 cases of Wernicke encephalopathy linked to GLP-1 agonists—13 from FAERS, 1 from published literature, and 1 from the authors’ own medical center. Most involved semaglutide (8 of 15) or tirzepatide (6 of 15), and most were reported in 2023–2024 (14 of 15). Statistical analysis showed these medications were reported disproportionately often in connection with Wernicke encephalopathy compared to other drugs (Lev et al., 2026). A separate systematic review of published case reports, using PRISMA methodology, independently identified 6 cases of Wernicke encephalopathy following semaglutide treatment for obesity, with poor outcomes—including progression to Korsakoff syndrome or death—in several patients (Bidesie & Oudman, 2026). A third analysis using the WHO’s global safety database (VigiBase) identified 19 cases total, noting that 68% involved nausea, vomiting, or reduced food intake, with weight loss ranging from 3.5 to 13.3 kg per month (Gras et al., 2025). Note that these three data sources likely include overlapping individual cases rather than representing 40 distinct patients.
On the broader nutritional picture, a 2026 narrative review in Clinical Obesity pooled data from six studies covering 480,825 adults taking GLP-1 medications. Vitamin D deficiency was most common, affecting 13.6% of patients after 12 months; nutritional anemia affected 4%, iron deficiency 3.2%, and B-vitamin deficiency 2.6%. Notably, thiamine and B12 deficits specifically increased over time (Urbina et al., 2026). This same study was covered by Harvard Health Publishing in March 2026, with Dr. Robert H. Shmerling of Harvard Medical School advising that patients on GLP-1 medications should ensure adequate intake of vitamin D, iron, and B vitamins, and consider a daily multivitamin.
Warning Signs: Recognizing B1 Deficiency
Key symptoms of vitamin B1 deficiency to watch for.
Early Symptoms
Mild to moderate B1 deficiency typically develops gradually. Early warning signs include:
- Persistent fatigue disproportionate to your activity level
- Difficulty concentrating or brain fog
- Irritability or mood changes
- Peripheral neuropathy: tingling, numbness, or “pins and needles” sensations in feet or hands
- Muscle weakness or cramping
- Nausea unrelated to your GLP-1 medication
These symptoms often mimic other common conditions, so they’re easy to overlook—especially if you attribute them to your medication’s side effects or weight loss.
Advanced Symptoms (Wernicke Encephalopathy)
Severe B1 deficiency can progress to Wernicke-Korsakoff syndrome. Acute Wernicke encephalopathy is a medical emergency with symptoms including:
- Severe confusion or delirium
- Rapid eye movements (nystagmus)
- Loss of muscle coordination (ataxia)
- Vision problems or double vision
- Loss of consciousness in extreme cases
If you or someone you know taking GLP-1 agonists experiences these symptoms, seek emergency medical care immediately.
The Serious Complication: Wernicke-Korsakoff Syndrome
Wernicke encephalopathy is inflammation and damage to specific brain regions (mammillary bodies and parts of the thalamus) caused by severe thiamine deficiency. It’s a medical emergency.
If acute Wernicke encephalopathy is treated quickly with high-dose thiamine supplementation, the neurological damage may be partially or fully reversible. However, if treatment is delayed, the condition can progress to Korsakoff syndrome—a chronic form characterized by permanent memory loss, difficulty forming new memories, and confabulation (unconsciously filling memory gaps with invented information).
Korsakoff syndrome can be irreversible. Patients may lose years or decades of memories and struggle with basic tasks. The psychological and social consequences are profound.
How to Prevent B1 Deficiency If You Take Ozempic
Prevention is far easier than treating established deficiency. Here’s a practical approach:
A practical guide to preventing B1 deficiency while taking Ozempic.
1. Eat intentionally, even without appetite: Since appetite suppression is profound, make calories count. When you do eat, prioritize nutrient-dense foods, especially those rich in B1. Examples:
- Pork tenderloin
- Whole grain bread and cereals
- Almonds and other nuts
- Chickpeas and lentils
- Fish (especially trout)
- Nutritional yeast
2. Monitor with your healthcare provider: Ask your doctor about baseline thiamine levels before starting Ozempic, and schedule follow-up testing annually or whenever you experience suspicious symptoms.
3. Consider supplementation: Discuss with your physician whether a B-complex vitamin or thiamine supplement is appropriate for you. Standard supplementation is safe and inexpensive.
4. Report gastrointestinal symptoms: If nausea, vomiting, or diarrhea is severe or persistent, inform your doctor. Severe GI symptoms compound nutritional risk.
5. Know your risk factors: Risk is higher if you:
- Take high doses of semaglutide for obesity
- Have a history of poor nutrition or eating disorders
- Use alcohol (alcohol also impairs B1 absorption)
- Have had bariatric surgery (compound malabsorption risk)
What to Discuss With Your Doctor
During your appointments, bring up these specific questions:
- “Should I have thiamine levels checked before starting Ozempic and periodically during treatment?”
- “Are there specific B-complex vitamins you recommend for GLP-1 agonist users?”
- “What symptoms should I watch for that might indicate B1 deficiency?”
- “If I experience severe fatigue or numbness, should I contact you immediately?”
- “Are there any foods I should especially emphasize while taking this medication?”
- “How will we monitor my nutritional status?”
Your doctor may recommend:
- Baseline testing: Serum thiamine or RBC transketolase activity
- Periodic monitoring: Annual or as-needed blood work
- Supplementation: Typically a B-complex vitamin or standalone thiamine
- Dietary counseling: A registered dietitian familiar with GLP-1 side effects
Key Takeaways
- Ozempic and similar GLP-1 agonists can increase the risk of vitamin B1 (thiamine) deficiency through appetite suppression and gastrointestinal effects.
- Severe B1 deficiency can progress to Wernicke-Korsakoff syndrome, a potentially irreversible neurological condition affecting memory and coordination.
- Early symptoms include fatigue, brain fog, peripheral neuropathy, and mood changes; advanced symptoms are a medical emergency.
- Prevention through intentional nutrition, regular monitoring, and supplementation (when appropriate) is highly effective.
- If you’re taking Ozempic, discuss B1 monitoring and prevention with your healthcare provider.
- Seek emergency care immediately if you experience severe confusion, vision problems, or loss of coordination.
Frequently Asked Questions
Is B1 deficiency common with Ozempic?
No—it appears to be rare. Published case counts range from single digits to about 19 cases across different safety databases (which likely overlap), against millions of GLP-1 prescriptions worldwide. However, broader micronutrient deficiencies (vitamin D, iron, B-vitamins as a group) are more common, affecting single-digit percentages of users in pooled studies.
Can I take a B1 supplement without medical supervision?
Standard-dose thiamine or B-complex supplements are generally considered safe for most adults, but you should still tell your doctor, especially since B1 deficiency often occurs alongside other deficiencies (like B12 and folate, as seen in documented case reports) that may need separate evaluation.
Do I need testing before starting Ozempic?
There’s no universal medical guideline requiring baseline thiamine testing before starting Ozempic. However, given the documented risk, it’s reasonable to ask your doctor whether baseline and periodic monitoring makes sense for your individual situation, especially if you have risk factors like a history of poor nutrition, alcohol use, or prior bariatric surgery.
Sources
- Bidesie J, Oudman E. Wernicke’s Encephalopathy Following Semaglutide Treatment for Obesity: A Systematic PRISMA Review of Case-Based Evidence. Obesity. 2026. PubMed
- Lev D, et al. Glucagon-like peptide-1 receptor agonists and Wernicke encephalopathy: A pharmacovigilance study and literature review. Clinical Nutrition. 2026. PubMed
- Gras C, et al. Semaglutide-induced Wernicke encephalopathy: a comprehensive analysis. European Journal of Clinical Nutrition. 2025. PubMed
- Urbina J, Salinas-Ruiz LE, Valenciano C, Clapp B. Micronutrient and Nutritional Deficiencies Associated With GLP-1 Receptor Agonist Therapy: A Narrative Review. Clinical Obesity. 2026. PubMed
- Cohan CJ, Townley L, Ortega E. Acute Nutritional Axonal Neuropathy in the Setting of Semaglutide-Associated Gastrointestinal Intolerance: A Case Report. Case Reports in Neurology. 2026. PMC
- Hendley J (reviewed by Shmerling RH, MD). Study: Taking GLP-1 drugs may increase risk of key nutrient deficiencies. Harvard Health Publishing. 2026. Harvard Health
