What vitamins should you take on GLP-1? A monitoring guide

If you are taking Ozempic, Wegovy or any other GLP-1 medication, there is a good chance nobody has checked what the drug is doing to your nutrient levels. Over 22% of people who start a GLP-1 develop at least one nutritional deficiency within twelve months, and most were never tested before they began. A review published in the journal Nutrients in November 2025 argues the fix already exists, borrowed from a field that has monitored this exact problem for decades.

TL;DR

There is no official nutrient-monitoring protocol for GLP-1 users yet. Researchers propose adapting the schedule used after bariatric surgery: a baseline blood panel before you start, rechecks at 3, 6 and 12 months, then yearly, plus a daily multivitamin and targeted top-ups for whatever runs low. Vitamin D, B12, iron, calcium, magnesium and protein are the ones to watch.

The blind spot in GLP-1 care

These drugs work by cutting how much you eat. That is the point, and it is also the risk. Food is where nearly all your micronutrients come from, so a medication that reduces intake by a quarter or more, for a year or longer, quietly reduces the vitamins and minerals arriving with it. Yet unlike most long-term treatments, there is no standard lab schedule attached to a GLP-1 prescription.

The authors of the Nutrients review put it plainly: no evidence-based monitoring protocol built specifically for GLP-1 users currently exists. Most prescriptions are written, titrated and refilled without a single nutrient panel. You often find out something is off only when symptoms show up, fatigue, hair shedding, muscle weakness, and by then the gap has been building for months. This is one category of GLP-1 side effects that is almost entirely preventable, because it moves slowly enough to catch with a cheap blood test.

What the deficiency data actually shows

The clearest numbers come from a retrospective study of 461,328 adults starting GLP-1 therapy. Within twelve months of starting:

• Over 22% had developed at least one nutritional deficiency
• Vitamin D was the most common, showing up in 13.6%
• Thiamine (vitamin B1) and other B vitamins followed
• Iron, calcium, magnesium and potassium appeared repeatedly

The trajectory matters as much as the total. Deficiency rates roughly double between month 6 and month 12, which we covered in detail in our breakdown of how nutrient gaps nearly double between month 6 and 12. The practical read is counterintuitive: the longer you are on the medication, the more the monitoring matters, not less.

Protein deserves its own line. Roughly a third of the weight lost on GLP-1 medications is lean tissue rather than fat, and the review found most users fall short of the protein needed to protect muscle, which it puts at 1.2 to 2.0 g per kg of body weight per day. Muscle loss on Ozempic and the rest of the class is not something you feel until strength is already gone, which is why it belongs in the monitoring conversation right next to the vitamins.

Why bariatric surgery is the right template

It sounds like a stretch until you look at the mechanism. Sleeve gastrectomy and GLP-1 medications reach the same destination by different roads: both cut food intake sharply and keep it cut. Bariatric surgery has spent two decades building nutrient-monitoring guidelines around precisely that problem, complete with baseline labs, follow-up intervals and default supplements. The review's argument is simple: GLP-1 users face a milder version of the same intake restriction, so the surgical playbook is a sensible starting point rather than something to invent from scratch.

Several surgical bodies already publish these schedules. The British Obesity and Metabolic Surgery Society (BOMSS, 2020), for example, checks ferritin, folate, B12, vitamin D, calcium, zinc and copper before surgery, then again at 3, 6 and 12 months, then annually. The American ASMBS and European ESPEN guidelines use similar panels. That timing is the part worth borrowing.

A monitoring schedule you can actually use

Here is the surgery-derived schedule translated for a GLP-1 user. None of this is a formal guideline yet, so treat it as a conversation to have with your prescriber, not a rule to self-apply.

When	What to check
Before you start (baseline)	Vitamin D, B12, folate, ferritin and iron, calcium, magnesium, plus a basic metabolic panel
3 months	Recheck anything low at baseline, plus vitamin D and B12
6 months	Vitamin D, B12, ferritin and iron, magnesium
12 months	Full repeat of the baseline panel
Annually after that	Full panel, plus a body-composition check if you can get one

The single most useful item is the baseline. If you never test before starting, you can never tell whether a low reading was caused by the medication or was already there. A large share of people begin GLP-1 therapy with a vitamin D or iron level that is already borderline, and shrinking portions only pull it lower.

Add one measure that is not a blood test: some record of muscle. A DEXA scan is the gold standard, but even grip strength, or a simple note of how many push-ups or sit-to-stands you can do today, gives you a baseline to defend over the next year.

What to take, and roughly how much

Testing tells you what is happening. Supplementation is what you do about it. The review's default, again borrowed from bariatric care, is a daily multivitamin for everyone plus targeted repletion for whatever the labs flag. Here is the practical shortlist. The amounts below are typical general ranges, not prescriptions, so set yours against your own results and your clinician's advice.

• A complete daily multivitamin, the baseline that covers thiamine, zinc, copper, selenium and the rest in one go
• Vitamin D, the most commonly deficient nutrient in this group, which often needs more than a multivitamin provides when your level is already low
• Vitamin B12, which falls as food intake drops and shows up as fatigue and brain fog. The link between vitamin B12 and semaglutide is well enough established to warrant a recheck at three months.
• Iron, especially for menstruating women, since low ferritin drives both fatigue and hair shedding
• Magnesium, commonly low even before treatment and easy to overlook
• Calcium alongside vitamin D for bone density, which rapid weight loss on GLP-1 medications can reduce
• Protein at 1.2 to 2.0 g per kg per day, paired with resistance training two or three times a week, which protect muscle far better than any capsule
• Omega-3 fats, hard to reach from the small portions of food you are now eating

This is the exact gap GLP-1 Shield is built to close: the specific nutrients that drain away when a medication cuts your intake, in the amounts the research points to, so the energy, hair and muscle you are working to keep are not quietly funded out of your own reserves. It does not replace testing. It gives you a sensible floor to stand on while you get tested.

How to bring this to your prescriber

You do not need to wait for official guidelines to act. A short, specific request works better than a vague worry. Try this: "I would like a baseline nutrient panel before I titrate up, and a recheck at three months." Bring the list, vitamin D, B12, folate, ferritin, calcium, magnesium. If your clinician has not seen the deficiency data, the bariatric-monitoring parallel is the fastest way to explain why it matters. Most will agree once the reasoning is on the table, because the tests are inexpensive and the downside of skipping them is not.

One honest caveat on the evidence. The review notes that most existing GLP-1 nutrition studies are short-term, observational, or based on food-recall questionnaires, which tend to miss the quieter, subclinical deficiencies. In other words, the real gap is probably wider than the 22% figure suggests, not narrower. That is an argument for testing sooner, not for waiting until the science is tidy.

Frequently asked questions

What blood tests should I get on GLP-1 medications?

A sensible baseline panel covers vitamin D, B12, folate, ferritin and iron, calcium, magnesium, and a basic metabolic panel, ideally before you start. Recheck the ones that came back low, plus vitamin D and B12, at 3 months, then repeat the full panel at 12 months and yearly after that. None of this is an official guideline yet, so raise it with your prescriber.

How often should I check my vitamin levels on Ozempic or Wegovy?

Adapting the schedule used after bariatric surgery, the reasonable intervals are before starting, then at 3, 6 and 12 months, then once a year. Check more often if a level comes back low or you develop symptoms like fatigue, hair loss or muscle weakness.

Do I need a multivitamin if I take semaglutide or tirzepatide?

A daily multivitamin is a reasonable default because food intake, and the micronutrients that come with it, drops sharply on these drugs. It does not replace testing, though. Nutrients that are already low, like vitamin D or iron, usually need a larger targeted dose than a multivitamin delivers.

Should I supplement even if I feel fine?

Deficiencies build quietly, and symptoms lag months behind the actual drop in your levels. Feeling fine at month 3 does not mean your stores are still full at month 12. Testing is the only way to know for sure, and a daily multivitamin plus adequate protein is a low-risk floor to stand on while you wait for results.