A rare and potentially life-threatening brain disorder is being linked to popular GLP 1 weight loss drugs after researchers identified cases involving severe vitamin B1 deficiency in patients using medications such as Ozempic, Wegovy, Mounjaro and Zepbound.
The condition, known as Wernicke encephalopathy, develops when the body becomes dangerously low in thiamine, also known as vitamin B1. The deficiency can interfere with normal brain and nerve function and may cause confusion, problems with balance and coordination, abnormal eye movements, and other neurological symptoms.
If the condition is not treated quickly, it can cause permanent brain damage and, in severe cases, death.
Researchers identified 15 cases involving people taking GLP 1 medications containing semaglutide or tirzepatide, the active ingredients used in some of the most popular diabetes and weight loss drugs on the market.
Eight cases involved semaglutide, which is found in Ozempic and Wegovy, while six involved tirzepatide, the active ingredient in Mounjaro and Zepbound. Most of the cases were reported during 2023 and 2024.
The findings are drawing attention because 13 of the 15 patients also experienced serious gastrointestinal or nutritional issues, including vomiting, significant appetite loss, malnutrition, or rapid weight loss.
Those complications may matter because GLP 1 drugs work partly by suppressing appetite and slowing digestion. For some patients, eating substantially less food over an extended period can reduce their intake of essential vitamins and nutrients.
Persistent vomiting can make the problem worse.
Thiamine is especially important because the body needs it to convert food into energy and support normal function of the brain and nervous system. Foods containing vitamin B1 include pork, fish, beans, lentils, whole grains, fortified cereals, nuts and seeds.
When patients are eating very little, rapidly losing weight, or repeatedly vomiting, their nutritional intake can drop enough to create deficiencies.
Researchers are not saying that GLP 1 medications directly caused every case of Wernicke encephalopathy.
Instead, the study raises concern that some of the effects associated with the medications, particularly severe appetite suppression, prolonged vomiting, and major weight loss, may create conditions that increase the risk of vitamin B1 deficiency in certain patients.
The FDA Adverse Event Reporting System accounted for 13 of the cases reviewed by researchers. Another case had previously appeared in medical literature, while one came from the researchers’ medical center.
FDA adverse event reports are designed to identify potential safety concerns, but they cannot prove that a particular medication caused a health condition. The reports can also include incomplete information, duplicate submissions, and patients with other underlying medical problems.
Still, researchers said the pattern is important enough for doctors to recognize.
Wernicke encephalopathy is traditionally associated with three major warning signs: mental confusion, problems with balance or coordination, and abnormal eye movement.
However, patients do not always experience all three symptoms at the same time.
Among the cases examined in the study, 11 patients displayed traditional symptoms of the disorder, but only two showed the complete three-symptom combination.
That can make the condition harder to recognize, particularly if symptoms initially appear unrelated.
Researchers also found concerning long-term outcomes. Among the 11 patients whose follow-up information was available, seven experienced lasting neurological complications.
Doctors consider Wernicke encephalopathy a medical emergency because early treatment can significantly affect the outcome.
Treatment typically involves rapidly replacing thiamine, often through intravenous or injectable doses when doctors suspect a serious deficiency.
Without treatment, patients can develop permanent memory and neurological problems. In some cases, Wernicke encephalopathy can progress into Korsakoff syndrome, a chronic condition involving significant memory impairment.
The findings come as GLP 1 medications continue to reshape the treatment of obesity and Type 2 diabetes.
Semaglutide and tirzepatide have become especially popular because of their ability to produce significant weight loss while also improving blood sugar control in appropriate patients.
However, nausea, vomiting, diarrhea, constipation, and reduced appetite are already among the commonly discussed gastrointestinal side effects associated with the drug class.
The new research adds another layer to the conversation by examining what can potentially happen when reduced appetite or gastrointestinal problems become severe enough to interfere with proper nutrition.
A separate analysis published in 2025 also examined reports involving semaglutide and Wernicke encephalopathy. Researchers in that analysis found that nausea, vomiting or reduced food intake appeared in a large share of the reported cases.
That does not mean most people taking GLP 1 medications are at risk of developing the rare disorder.
The number of identified cases remains extremely small compared with the millions of patients who have used the medications.
Instead, researchers are calling attention to specific warning signs that could signal a nutritional problem before it becomes more serious.
Patients experiencing persistent vomiting, severe appetite loss, unusually rapid weight loss, or symptoms such as confusion and trouble walking may require medical evaluation.
The findings are also not a recommendation for patients to suddenly stop taking prescribed GLP 1 medications. Medication changes should be discussed with a healthcare provider.
What the study does highlight is the importance of nutrition while taking medications capable of dramatically reducing appetite.
As GLP 1 shots continue helping patients shed significant amounts of weight, researchers are increasingly looking beyond the number on the scale and examining whether some patients may also be losing essential nutrients along the way.
