Thiamine is stored in the body for a matter of weeks, which is short compared with how long someone can spend eating very little and vomiting. A systematic review searched four databases for published cases of Wernicke’s encephalopathy after semaglutide prescribed for obesity, and found six [1]. The broader nutrient picture is in vitamins and minerals on a GLP-1.
The reason it is worth a reader’s attention anyway is the shape of the sequence, which was the same in every case. Prolonged gastrointestinal symptoms came first. Substantial and rapid weight loss followed. Only then did neurological deterioration appear, presenting as altered mental status and abnormalities of eye movement. The first two stages are visible to the person experiencing them, which makes this a rare harm with an early warning attached.
The outcomes are the reason not to soften any of this. Several cases did badly, including progression to Korsakoff syndrome, an irreversible amnesic disorder, and death. Wernicke’s is treatable, and treated early with parenteral thiamine the damage is largely preventable; left late it is not.
The practical translation is narrow and worth stating exactly. Persistent vomiting or an inability to keep food down for weeks is not a side effect to push through, and rapid weight loss on top of it compounds the problem rather than signaling success. That combination is a reason to contact a prescriber rather than to wait it out, and the dose-escalation period is when it is most likely to arise, as titration and nausea describes.
Nothing here is an argument for taking a supplement on your own initiative. Thiamine deficiency severe enough to reach the brain is a clinical emergency treated with injections under supervision, not something a tablet from a shelf addresses once symptoms have started. The general problem of eating much less while absorbing the same nutrients is the one running underneath, and it sits beside the muscle question in what these drugs do to muscle and the broader tolerability picture in the network meta-analysis of nineteen drugs.