Vitamin D on Ozempic and other GLP-1s: why it slips, and what a day of it costs

If you're on a GLP-1 medication — semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound) — the whole point is that you eat less. That's also the catch for nutrition: eat less food, take in fewer nutrients, and a few of them start to slip. In the data so far, the one that slips most often is vitamin D.

Why vitamin D is the one that slips

A 2026 review of nutritional deficiencies in people taking GLP-1 receptor agonists (Urbina and colleagues, Clinical Obesity) pulled together data on nearly half a million adults. Its headline finding, widely covered since: vitamin D was the most common deficiency, affecting roughly 14% of users after about a year, followed by low iron and B-complex vitamins.

The mechanism is straightforward. GLP-1 drugs suppress appetite and slow the stomach, so people eat less and absorb differently — and the population using them (often with obesity or type 2 diabetes) is already more likely to start out low on vitamin D. It's worth being clear about what the review is and isn't: it's largely observational, so it shows an association, not proof that the medication causes the deficiency. But the pattern is consistent enough that the study's authors called for nutritional assessment to be part of GLP-1 care.

The sensible numbers (and the one trap)

You don't need a special "GLP-1 dose" of vitamin D. The reference points that matter come from the NIH Office of Dietary Supplements:

• RDA: 600 IU (15 mcg) a day for most adults, 800 IU (20 mcg) over age 70.
• Tolerable Upper Limit: 4,000 IU (100 mcg) a day for adults.

The trap is assuming more is better. Vitamin D is fat-soluble and builds up; too much drives blood calcium too high, which is genuinely dangerous — more so if kidney function is reduced. That's why the honest answer to "how much should I take" is: get a blood test and let your prescriber set the dose. A number that's right for someone who tested at 15 ng/mL is wrong for someone at 40.

Where cost actually comes in

Here's the good news, and the part nobody else puts a number on: vitamin D is one of the cheapest things in the supplement aisle. Once your clinician has told you a target, the only question left is which product delivers it for the least money — and that's pure arithmetic.

The catch is that vitamin D softgels range from 400 IU to 10,000 IU, so price per bottle tells you almost nothing. A "365-count, 5,000 IU" bottle and a "60-count, 1,000 IU" bottle aren't remotely comparable on the shelf. The fair unit is cost per 1,000 IU — how much a standard chunk of vitamin D actually costs you. Across the catalog the same 1,000 IU can cost several times more from one brand to the next, and the priciest option is rarely the best-absorbed; it's usually just packaged better.

We rank every vitamin D we track by exactly that — real cost per dose — so you can hit whatever target your clinician set for the least money. The cheapest options are usually a fraction of a cent to a couple of cents per 1,000 IU.

If you take it with something else

Two pairings come up a lot for GLP-1 users, and both are on our cost-per-dose data:

• Vitamin D + magnesium — magnesium is a cofactor for activating vitamin D and is itself commonly low on GLP-1s.
• Vitamin D + K2 — often sold and taken together for how the two interact with calcium.

Neither is a rule, and supplements can interact with each other and with medications — so if you're building a stack on top of a GLP-1, run it past your clinician or pharmacist first. What we can tell you is the cost, so a fuller routine doesn't have to mean overpaying.

Try it: the GLP-1 nutrition calculator turns this into a priority-ranked list for your appetite level and diet — protein, vitamin D, B12, magnesium, electrolytes — each by cost per dose.

This is general shopping and quality guidance, not medical advice. Vitamin D needs are individual, high doses carry real risks, and anyone on a GLP-1 medication should decide on supplements with the clinician managing their treatment — ideally after a blood test.