Vitamin B1 (Thiamine) + B12

Beyond the basic roles of these vitamins, it helps to understand how GLP-1 medications physically change the way your body handles them. Because these drugs slow gastric emptying (meaning food leaves your stomach more slowly) and reduce the production of stomach acid, they can get in the way of Vitamin B12 being absorbed — and B12 needs a specific acid level plus a protein called intrinsic factor (a protein your stomach makes that helps pull B12 out of your food) to be released in the first place. This risk stacks even higher for the many users who also take Metformin or proton pump inhibitors (PPIs) for acid reflux, since both of these common medications are clinically proven to drain your B12 stores further. For this reason, many clinicians prefer injectable B12 — a shot that skips the slowed digestive tract entirely so the vitamin reaches your bloodstream.

The risk around Thiamine (B1) is especially time-sensitive — your body's stores can run low in as little as a few weeks if you're hit with persistent vomiting, a common side effect of GLP-1 therapy. Paradoxically, as you lose weight quickly, your body's demand for thiamine actually goes up even while your appetite is at its lowest. There's also a vital clinical safety point here: if a severe deficiency is suspected, giving glucose or a high-carbohydrate meal before replenishing thiamine can dangerously speed up the onset of life-threatening neurological complications.

Finally, some users need to be more aggressive with their monitoring. People with a history of bariatric surgery face a "perfect storm" of risk factors because their digestive systems are already malabsorptive (meaning they don't absorb nutrients well) before adding a GLP-1 medication. If you're in a high-risk category or noticing neurological symptoms like balance issues or confusion, a standard blood test may not be enough — experts recommend measuring thiamine diphosphate (the active form your cells actually use) specifically in the red blood cells for a truly accurate read on your status. Using high-potency, bioavailable forms of these B vitamins isn't just about energy — it's a critical safeguard against permanent nerve and brain damage.