Vitamins and Minerals When You Eat Very Little on Ozempic, Zepbound or Mounjaro: What to Watch

On semaglutide (Ozempic, Wegovy) or tirzepatide (Mounjaro, Zepbound) you eat a lot less, and with less food it gets easier to fall short on some vitamins and minerals. A 2025 advisory from four US obesity and nutrition societies names iron, calcium, magnesium, zinc and several vitamins, including D and B12. It doesn’t say everyone needs a supplement: it calls for nutrient-dense food, monitoring, and specific supplements considered person by person^[1]. Here I cover what to watch, which foods supply it, what lab tests your doctor may consider, and how to read a supplement label.

What the 2025 advisory says

The 2025 advisory from four US societies, including The Obesity Society and the Obesity Medicine Association, has a section on nutrient deficiencies. Its positions^[1]:

• Why there’s a risk. Calorie intake drops by 16% to 39% compared with placebo, and the risk rises below 1,200 calories a day for women and 1,800 for men. Digestive side effects can further compromise absorption, and many people with obesity start with existing shortfalls.
• Nutrients it names as examples. Iron, calcium, magnesium and zinc, plus vitamins A, D, E, K, B1 (thiamin), B12 and C.
• What it recommends eating. Fruits, vegetables, whole grains, legumes, lean proteins, nuts and seeds, minimally processed.
• On supplements. They “can be proactively considered for at-risk nutrients, such as vitamin D, calcium, B12, or a multivitamin-mineral tablet,” at appropriate doses and tailored to each person. It sets no doses.
• On lab tests. Before starting, testing may be indicated after a very low-calorie diet, bariatric surgery, celiac disease, other inflammatory conditions or a prior deficiency. During treatment, it calls for reassessing diet and nutrient levels.

How often does it happen? A 2025 study of insurance claims from 461,382 adults who started a GLP-1 between 2017 and 2021, 80.5% with type 2 diabetes, found that 22.4% had a nutritional deficiency diagnosis within a year. Vitamin D was the most common (13.6%). Read it with care: these are billing codes with no detail on how deficiencies were measured, and the study was funded by Abbott^[9], which sells nutrition products.

Nutrient by nutrient: how much and where to find it

The table shows two numbers. The RDA (Recommended Dietary Allowance) from the NIH Office of Dietary Supplements is what covers nearly all healthy adults. The Daily Value (DV) is the fixed number that Nutrition Facts and Supplement Facts labels use for “% DV,” set by the FDA^[19].

RDAs from the NIH fact sheets linked below^{[10,11,12,13,14,15,16,17]}. mcg: micrograms. DFE: dietary folate equivalents. Per-serving numbers from USDA FoodData Central^[21], on our food pages; we don’t have per-food data for vitamin D, folate or zinc.

Two things stand out. Iron’s DV (18 mg) is set at the RDA for women 19 to 50^[11,19], so a product with “100% DV” iron gives men and women over 50 more than twice their RDA. And the NIH says there’s no RDA for EPA and DHA, the omega-3s in fish^[17].

Iron. According to the NIH Office of Dietary Supplements, your body absorbs plant iron better alongside meat, poultry, seafood or vitamin C foods such as bell peppers, tomatoes or citrus. Iron deficiency anemia brings tiredness, lack of energy, GI upset and trouble concentrating^[11]. See the iron-rich foods ranking or cooked lentils.

Vitamin B12. It occurs naturally only in animal foods; plant foods have it only if fortified. The NIH lists the signs of deficiency as tiredness, weakness, pale skin, numb or tingly hands and feet, and balance or memory problems. Adults over 50, vegans, people who’ve had stomach or intestinal surgery, and people on metformin or acid reducers such as omeprazole are at higher risk^[10]. When appetite is low, shellfish goes a long way: 100 g of mussels is five times the RDA^[10,21]. More in the vitamin B12 foods ranking.

Calcium and vitamin D. The advisory names both among the supplements to consider and stresses protecting bone during weight loss^[1]. Calcium is in dairy, tofu, fortified plant milks such as soy milk, and canned fish with bones such as sardines in oil^[12]; see the calcium-rich foods ranking. Vitamin D is in few foods. The NIH notes that people with obesity or who’ve had gastric bypass may need more than other people^[13].

Folate. It isn’t on the advisory’s list, but a 2026 review includes it in baseline labs and among the nutrients to watch when planning a pregnancy^[7]. It’s in leafy greens, beans and oranges, per the NIH^[14]. If you’re planning a pregnancy, talk to your doctor: these drugs’ labels have their own pregnancy guidance^[2,3,4,5].

Magnesium and zinc. Magnesium is in beans, nuts, seeds and leafy greens^[15]: 25 g of pumpkin seeds is over 40% of a woman’s RDA^[15,21] (magnesium-rich foods ranking). Zinc is highest in oysters, and meat, fish, poultry and seafood are good sources; beans, nuts, whole grains, eggs and dairy also provide some, per the NIH^[16].

Omega-3. The advisory doesn’t list it as a nutrient at risk^[1]. A serving of mackerel has 2.8 g of EPA plus DHA^[21] (omega-3 foods ranking).

When you eat little, foods that cover several nutrients at once pay off: shellfish (iron, B12, zinc), canned sardines with bones (calcium, B12, omega-3) or beans (iron, magnesium, folate). The weekly menu planner can help, and potassium, which the advisory doesn’t list, has its own potassium-rich foods ranking.

Vitamin B12 and metformin

Many people take a GLP-1 alongside metformin. I searched the US prescribing information for Ozempic, Wegovy, Mounjaro and Zepbound for “B12” and “cobalamin”: none of them mentions vitamin B12^[2,3,4,5].Dotted underline: a drug-label fact; hover over it to see the date it was checked.

Metformin is a different story. The American Diabetes Association (ADA) calls lower B12 levels “a known consequence of long-term treatment with metformin.” Its 2026 Standards recommend considering periodic B12 checks in people on long-term metformin, especially with anemia or peripheral neuropathy (recommendation 3.10). The text highlights doses of 1,500 mg a day or more, 4 to 5 years of treatment and suspected deficiency, for example with anemia, neuropathy or chronic kidney disease^[6]. The NIH adds that metformin might reduce B12 absorption^[10].

The GLP-1 labels don’t describe any B12 effect of their own^[2,3,4,5]. What can add up is eating less meat, fish, eggs and dairy.

Do you need “bariatric vitamins”?

After weight-loss surgery, guidelines call for lifelong supplements and regular lab checks, because surgery reroutes the digestive tract and, in some procedures, impairs absorption^[7,8].

The 2026 review is direct: those principles shouldn’t be applied automatically to people on a GLP-1, because the drug doesn’t bypass the stomach or small bowel and doesn’t cause surgical malabsorption. The risk comes from eating less, nausea and food aversions. It proposes assessing and supplementing based on what each person eats, their symptoms, other conditions, any past surgery and a confirmed deficiency^[7].

A 2025 review with overlapping authors goes further: it suggests slower gastric emptying might affect absorption and calls for borrowing some bariatric monitoring, while admitting that many studies are short-term, observational or based on dietary recall^[8]. My read: the main driver is eating less.

The exception: if you’ve already had bariatric surgery and now take a GLP-1, you still need your post-surgery follow-up, and eating even less can worsen existing deficiencies.

Does a GLP-1 block vitamin absorption?

The labels talk about oral medicines, not vitamins. Zepbound’s says tirzepatide delays gastric emptying and has the potential to impact the absorption of oral medications taken at the same time, with monitoring advised for narrow-therapeutic-index drugs such as warfarin^[3]. Wegovy’s says in clinical pharmacology trials with semaglutide 1 mg injection, semaglutide did not affect the absorption of oral medications^[2]. None of the four labels mentions vitamins or supplements in its drug-interaction section^[2,3,4,5].

What the Medication Guides do say is to tell your healthcare provider about all the medicines you take, including over-the-counter medicines, vitamins and herbal supplements^[2,3].

Wegovy tablets are a special case. The label says to take them on an empty stomach with up to 4 ounces of water and wait at least 30 minutes before eating, drinking or taking other oral medications^[2]. If you take a supplement pill, ask your pharmacist how to fit it in.

Lab tests your doctor may order

There’s no official guideline on labs for people on a GLP-1. There are proposals:

• The 2026 narrative review proposes, at baseline and “where appropriate,” a complete blood count, ferritin (your iron stores), folate and vitamin D, repeated as needed. After bariatric surgery or with conditions that affect nutrition, such as celiac disease, it adds B12 and calcium; after surgery, also zinc, copper and selenium^[7].
• The ADA: periodic B12 with long-term metformin^[6].
• The 2025 advisory: testing before starting when there are risk factors, and reassessment during treatment^[1].

Your doctor decides what to order and when. To bring your questions ready, use the doctor-visit checklist, and log signs like the ones the advisory lists in the symptom diary.

When a supplement makes sense

The NIH multivitamin fact sheet says people on low-calorie diets, with a poor appetite or who avoid certain foods “might consider taking an MVM.” It adds that a basic one is unlikely to cause harm, but that combined with other supplements or fortified foods it can push intake of iron, vitamin A, zinc, niacin or folic acid too high^[18].

Interactions to know, per the NIH:

• Iron and calcium can reduce the absorption of levothyroxine (for thyroid); calcium can also interfere with some antibiotics^[11,12].
• Magnesium can hinder some antibiotics and bisphosphonates (for osteoporosis)^[15].
• The vitamin K in some multivitamins changes how warfarin works^[18].
• High doses of omega-3s may cause bleeding problems with warfarin^[17].
• Omeprazole and similar acid reducers lower B12 and iron absorption^[10,11].

If you take other medicines, check with your doctor or pharmacist before adding anything.

Magnesium has a second use: the advisory lists magnesium citrate among options for constipation when diet isn’t enough^[1]. There it works as a laxative, and the NIH warns that high intakes from supplements cause diarrhea, nausea and cramping^[15]. More in constipation on GLP-1s.

How to read a supplement label in the US

• Not FDA-approved. According to the FDA, it has no authority to approve dietary supplements before they’re marketed^[20].
• The disclaimer. A claim like “supports bone health” is a structure/function claim, and by law it must carry: “This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.”^[20]
• Check the % DV against the table above, especially for iron.

One last thought. If you can barely eat at all, the problem isn’t which pill to take but that food is missing, and that needs a conversation with your doctor. And if eating less has become a goal in itself, or food brings anxiety or guilt, reach out for help: eating disorders are serious and treatable, as MedlinePlus notes^[22]. The full eating plan is in what to eat on GLP-1s.

Frequently asked questions

Do I need to take a multivitamin on Ozempic?

It isn’t required, and no label asks for it^[2,3,4,5]. The 2025 advisory says one can be considered person by person^[1]. If you’re eating very little or cutting out food groups, bring it up with your doctor.

Is my hair falling out because of low vitamins?

It can play a part, but the most common cause after weight loss is telogen effluvium, a temporary shedding. The advisory lists excessive hair loss among the signs of deficiency, alongside fatigue or flaky skin^[1]. I cover what helps and what doesn’t in hair loss on GLP-1s.

My vitamin D came back low. Is it the Zepbound?

The lab result alone can’t tell you. Low vitamin D is common with obesity or little sun^[13], and it may have been low before you started. In the study of 461,382 people it was the most common deficiency diagnosis^[9], but that doesn’t prove the drug caused it. Your doctor will decide whether and how to treat it.

Can I take magnesium with Ozempic?

The labels say nothing about magnesium^[2,3,4,5]. As covered above, high intakes cause diarrhea, and it can hinder some antibiotics and bisphosphonates^[15]. Ask your doctor or pharmacist if you take other medicines.

Should I take my vitamins at the same time as my shot or pill?

For the injections, the labels say nothing about vitamins^[2,3,4,5]. For Wegovy tablets, the label says to wait at least 30 minutes before taking other oral medications^[2]. Confirm the timing with your pharmacist.