People assume the biggest risks of drugs like Ozempic and Zepbound are nausea and the monthly bill. The medical literature is starting to describe a rarer one, and it runs through a vitamin most people never think about: B1.
These medications belong to a class called GLP-1 (glucagon-like peptide-1) drugs. Semaglutide is sold as Ozempic and Wegovy; tirzepatide is sold as Mounjaro and Zepbound. They work partly by cutting appetite, which is the whole point. But a small number of published case reports now tie them to Wernicke encephalopathy, a serious brain emergency caused by running low on thiamine, which is vitamin B1. Here is what the evidence actually says.
Can Ozempic and other GLP-1 drugs cause a vitamin B1 deficiency?
Not directly, and not for most people. A small but growing set of published case reports links GLP-1 drugs, including semaglutide and tirzepatide, to thiamine deficiency severe enough to cause Wernicke encephalopathy. This is an association clinicians are starting to watch, not a proven cause, and it appears rare.
The reports come from several teams (Bidesie and Oudman, 2026; Gras and colleagues, 2025; Lev and colleagues, 2026). In each, the pattern was similar. The drug did not appear to attack thiamine on its own. Instead, weeks of eating very little, or repeated vomiting, drained the body's supply. The deficiency does the damage; the drug just sets the stage.
Why would an appetite drug affect thiamine at all?
Because thiamine runs out fast. Vitamin B1 is water-soluble, and the body stores only about two to three weeks' worth. GLP-1 drugs cut appetite and can cause nausea and vomiting. Weeks of eating little or throwing up can empty a reserve that was never built to last long.
In the published cases, the timeline was consistent. Prolonged nausea, vomiting, reduced eating, and fast weight loss came first. The neurologic symptoms came later. Reduced intake or vomiting was documented in about two-thirds of the cases (Gras and colleagues, 2025). The main effect of these drugs, a smaller appetite, is exactly what sets that stage.
What is Wernicke encephalopathy, and how do I recognize it?
Wernicke encephalopathy is a medical emergency caused by a sharp drop in thiamine that starves the brain of fuel. It classically shows up as some mix of three things: confusion, eye-movement or vision problems, and an unsteady walk. Not everyone gets all three. It needs treatment within hours, not days.
Oral vitamin B1 does not absorb fast enough to reverse an active case. That is why hospitals give it through an IV. The goal is to flood the brain with thiamine before the shortage does lasting harm. Caught early, people often recover well; caught late, the damage can be permanent.
How common is this, really?
Rare. Tens of millions of people use GLP-1 drugs, and the total number of reported Wernicke cases worldwide is small by comparison. Safety databases do show it turning up more often than expected, which is a signal worth studying. That is not the same as proof the drugs cause widespread deficiency.
Two large adverse-event reporting systems track this: the FDA's FAERS (the U.S. Food and Drug Administration's public log of side-effect reports) and the WHO's VigiBase (the World Health Organization's global version). Both show Wernicke encephalopathy reported more often than expected in people on these drugs (Lev and colleagues, 2026). One analysis put the reporting odds at roughly 2.3 times expected. In safety terms that is a "signal": a pattern that earns a closer look, not a verdict.
Who should pay closer attention?
Anyone on a GLP-1 drug with persistent vomiting, very low food intake, or rapid weight loss. People with obesity already start with higher-than-average rates of low thiamine, from roughly 7 percent up to about a third in some groups, before any medication. Add weeks of poor intake and the margin narrows.
That baseline matters. Studies find low thiamine in people with obesity even before treatment, ranging from about 7 percent to nearly a third depending on the group studied (systematic review 2023; Costello and colleagues, 2025). Experts now suggest clinicians watch for thiamine deficiency in GLP-1 patients with persistent vomiting or rapid weight loss, the same way nutrition is tracked after bariatric (weight-loss) surgery.
What to do
None of this is a reason to fear a GLP-1 drug that is helping you. It is a reason to pay attention to the ordinary signals your body sends. If you are vomiting often or eating almost nothing for weeks, that is worth a conversation with your prescriber, not silence.
- Tell your prescriber if nausea, vomiting, or a near-total loss of appetite lasts more than a few days. Persistent is the key word.
- Do not push through weeks of eating almost nothing. Fast weight loss with little food coming in is the exact setup these case reports describe.
- Memorize the three red flags of Wernicke encephalopathy: new confusion, vision or eye-movement changes, and an unsteady walk. If any appear, treat it as an emergency and go to the ER.
- Ask your clinician whether your thiamine level should be checked if you have had a long stretch of vomiting or barely eating.
- Do not try to self-treat a suspected case with over-the-counter B1 pills. An active case needs IV thiamine in a medical setting.
The appetite these drugs turn down is also the body's oldest alarm.