Energy intake can fall by up to 40% on these medications. Nutrients do not get an exemption from that, and the foggy end of the symptom list is where shortfalls tend to show.
The mechanism is straightforward and the evidence is still catching up. A 2025 review found that clinical use of GLP-1 receptor agonists reduces energy intake by up to 40%, and flagged that this raises concern about adequate dietary intake and an increased risk of micronutrient deficiencies.2 The same review explicitly calls for more research, so this is a well-reasoned concern rather than a documented epidemic. B12 matters here because low B12 is an established cause of fatigue and cognitive symptoms, which makes it worth testing rather than guessing.
Weight loss is the goal, so a big drop in intake looks like success. Nutrients come along for the ride either way.
A 2025 review searching six databases to June 2025 reported that clinical use of GLP-1 receptor agonists reduces energy intake by up to 40%, producing weight loss well beyond what lifestyle changes typically achieve. The authors note that this significant reduction in food consumption raises concerns about adequate dietary intake, with an increased risk of micronutrient deficiencies.2
That is a mechanistic concern stated by researchers, and it is worth reading precisely. The review's purpose was to summarise current evidence and highlight the need for further research.2 Nobody has shown that most people on these medications become deficient. What they have shown is that intake falls far enough to make it a sensible thing to watch.
B12 earns its reputation because the symptoms of being short of it overlap almost perfectly with how people describe life on a GLP-1: tiredness, weakness, and problems with memory and concentration. If you are foggy and eating much less than before, it is a reasonable thing to ask about.
Two cautions keep this honest. First, the strong, well-established link between a diabetes medication and B12 is with metformin, not with GLP-1 medications, and many people take both, which muddies attribution. Second, B12 is stored in the liver and shortfalls develop slowly, so a few low-appetite weeks is a different proposition from a year of greatly reduced intake. A blood test settles it, and guesswork does not.
In practice, the shortfalls that make people foggy first are usually blunter than a specific vitamin:
A supplement cannot substitute for food, and it will not correct a nutrient gap it does not contain. For the separate question of focus, some people look at non-stimulant options rather than more caffeine. Lion's Mane (Hericium erinaceus) is a culinary mushroom examined in a small double-blind pilot trial tracking cognitive performance, stress, and mood in young adults.4 That is early evidence from a small field and it was not studied alongside these medications. If a nutrient gap is your problem, testing and food are the fix.
It is a flagged concern rather than an established fact. A 2025 review found these medications can reduce energy intake by up to 40% and warned this raises the risk of micronutrient deficiencies, while explicitly calling for more research. Testing is the way to know your own position.
Low B12 is an established cause of tiredness and problems with memory and concentration, so it is worth ruling out if you are foggy and eating much less. Note that the strong medication link for B12 is with metformin rather than GLP-1 drugs, and many people take both.
Ask your clinician rather than deciding alone. A multivitamin is a reasonable thing to discuss when intake has dropped substantially, but it is not a substitute for eating enough, and some nutrients, iron in particular, should not be taken speculatively.
It depends on the nutrient and your starting stores. B12 is stored in the liver and depletes slowly, so months of greatly reduced intake matter more than a few difficult weeks. Other shortfalls, especially protein and fluid, show up much faster.
That depends on your history, your other medications, and how long your intake has been reduced, so bring those details rather than a fixed list. B12 and iron studies are common starting points when fatigue and fog are the complaint.
Consult your prescriber. Do not start, stop, or change any prescription medication or add a new supplement without talking to the clinician who manages your care. This page is educational and is not medical advice, diagnosis, or treatment.
FDA disclaimer. Statements about dietary supplements have not been evaluated by the FDA. Supplements are not intended to diagnose, treat, cure, or prevent any disease.