Do You Need a Multivitamin on Semaglutide or Tirzepatide? A Doctor Explains

By Dr. Farhan Abdullah, DO | Medical Director, Magnolia Functional Wellness | Southlake, TX

You've been on semaglutide or tirzepatide for a few months. The weight's coming off, your clothes fit differently, and people are starting to notice. Then one morning you wake up feeling completely drained. Your hair looks thinner in the mirror. Your nails are brittle. You're cold all the time. And you're wondering: is this the medication, or is something else going on?

Here's what I tell patients at Magnolia Functional Wellness when they come in with these concerns: you're probably not absorbing (or consuming) enough of the nutrients your body needs. GLP-1 medications are remarkably effective at reducing appetite and caloric intake, but that very effectiveness creates a nutritional blind spot that almost nobody talks about when you're getting started.

I'm Dr. Farhan Abdullah, and this is the conversation I've been having more frequently with my weight loss patients in Southlake over the past year. It matters more than most people realize.

The Calorie Equation Has a Hidden Variable

When you eat 40% less food, which is roughly what most patients on GLP-1 medications end up doing, you're not just consuming fewer calories. You're consuming fewer vitamins, fewer minerals, less fiber, and less protein. That sounds obvious when I say it that plainly, but it catches people off guard because the focus during those first few months is entirely on the scale.

A 2025 retrospective study published in Obesity Pillars by Butsch et al. analyzed claims data from over 461,000 adults newly prescribed GLP-1 receptor agonists and found that over 22% had documented nutritional deficiencies within one year of starting treatment. Vitamin D deficiency was the most common, showing up in 7.5% of patients at six months and 13.6% at twelve months. And that's only the patients who were actually screened for it. The real numbers are almost certainly higher.

A 2026 narrative review in Clinical Obesity by Urbina et al. pulled data from nearly 481,000 adults and found similar patterns: iron depletion was frequent, with GLP-1 users showing 26 to 30% lower ferritin levels, and over 60% of users consumed below recommended amounts of calcium and iron. Average vitamin D intake was just 20% of the daily recommendation.

Those aren't trivial numbers. They represent real clinical consequences that show up as fatigue, hair loss, bone density concerns, mood changes, and weakened immunity. The medication didn't cause these deficiencies directly. The dramatically reduced food intake did.

Which Nutrients Are Most at Risk (and Why)

Not every vitamin and mineral carries the same risk on GLP-1 therapy. Some are particularly vulnerable because of how they're absorbed, what foods contain them, and how much you need relative to what you're eating.

Vitamin D tops the list consistently across the literature. It's already the most common deficiency in the general population (especially here in Texas, where you'd think we'd get enough sun). When caloric intake drops significantly, vitamin D intake drops with it because its dietary sources are limited: fatty fish, egg yolks, fortified dairy, and fortified cereals. If you were barely meeting your vitamin D needs before starting a GLP-1, you're almost certainly falling short now.

Iron is the second most common concern, particularly for premenopausal women who already have higher iron demands. Red meat is one of the best sources of heme iron, and it's one of the first foods people eat less of when their appetite tanks. The Urbina review noted ferritin drops of 26 to 30% in GLP-1 users, which is enough to push borderline patients into deficiency territory.

B vitamins, especially B12 and folate, deserve attention. B12 absorption requires adequate stomach acid and a protein called intrinsic factor. GLP-1 medications slow gastric emptying and can alter the gastric environment in ways that may affect B12 absorption over time. If you're also taking a proton pump inhibitor for reflux (which is common), the risk compounds. B12 deficiency is insidious: it develops slowly, and by the time you have symptoms like numbness, tingling, or cognitive fog, the deficiency has been present for months.

Calcium and magnesium round out the high-risk group. Calcium intake drops when dairy consumption drops, and magnesium (found in nuts, seeds, leafy greens, and whole grains) suffers when overall food volume shrinks. Both have implications for bone health, muscle function, and cardiovascular health that extend well beyond what you'd notice day to day.

When Deficiency Gets Serious: The Wernicke Warning

Most nutritional deficiencies on GLP-1 therapy are gradual and correctable. But there's a severe end of the spectrum that every prescribing physician and every patient should know about: thiamine (vitamin B1) deficiency leading to Wernicke encephalopathy.

A 2026 pharmacovigilance study by Lev et al. published in Clinical Nutrition identified 15 cases of Wernicke encephalopathy linked to GLP-1 receptor agonist use, most involving semaglutide or tirzepatide. The pattern was consistent: patients experienced significant nausea and vomiting (common GLP-1 side effects), couldn't keep food down for extended periods, and developed confusion, vision changes, and gait instability. These are neurological emergencies that result from the brain running out of thiamine.

A separate case report by Zahir et al. in BMC Neurology documented a 37-year-old woman who developed severe axonal polyneuropathy and Wernicke encephalopathy after rapid weight loss on semaglutide. She presented with progressive leg numbness, weakness, and vision problems. The cause? Thiamine deficiency from prolonged inadequate intake during aggressive weight loss.

This is rare. I want to be clear about that. But "rare" doesn't mean "ignorable," especially when it's entirely preventable. If you're experiencing persistent vomiting on a GLP-1, can't eat for days at a time, or are losing weight dramatically fast, your prescriber needs to know immediately. And a basic multivitamin with thiamine should be non-negotiable from day one.

So, Do You Actually Need a Multivitamin?

I'm generally not someone who pushes supplements on patients. The supplement industry is full of unnecessary products marketed through fear and pseudoscience, and I've seen plenty of patients spend hundreds of dollars a month on pills they don't need. But GLP-1 therapy is one of the situations where targeted supplementation makes genuine clinical sense.

Here's my practical approach at Magnolia Functional Wellness:

Baseline labs before starting. Every patient starting a GLP-1 at our clinic gets a comprehensive metabolic panel, CBC, iron studies (ferritin, TIBC, iron saturation), vitamin D (25-OH), B12, folate, and magnesium. You can't correct what you don't measure, and knowing your starting point tells us exactly where you're vulnerable.

A quality multivitamin from the start. Not because everyone will develop deficiencies, but because the probability is high enough that prevention is smarter than treatment. The key word is "quality." A dollar-store multivitamin with cheap oxide forms of minerals and inadequate doses isn't doing much. Look for methylated B vitamins (methylfolate, methylcobalamin), chelated minerals (magnesium glycinate, not magnesium oxide), and vitamin D3 at 2,000 to 5,000 IU depending on your baseline level.

Targeted supplementation based on labs. If your vitamin D is below 40 ng/mL, you need more than what a multivitamin provides. If your ferritin is dropping, we might add iron or consider IV iron infusions for faster correction. If your B12 is trending down, sublingual B12 or injections are more effective than oral tablets for many patients.

Recheck at 3 and 6 months. Lab monitoring isn't a one-and-done exercise. The deficiency patterns evolve as your caloric intake changes, your body composition shifts, and your weight stabilizes. What looks fine at month one might be trending toward deficiency by month four.

Protein Deserves Its Own Conversation

Protein isn't a vitamin or mineral, but it's arguably the most important nutritional gap on GLP-1 therapy. When you're eating dramatically less food and losing weight rapidly, your body breaks down both fat and muscle. The SURMOUNT trials showed that roughly a third of weight lost on tirzepatide was lean mass. That's not a medication side effect, exactly. It's a consequence of caloric restriction without adequate protein intake and resistance training.

My recommendation for patients on GLP-1 medications: aim for 0.7 to 1.0 grams of protein per pound of ideal body weight per day. For most of my patients, that means 80 to 130 grams daily. When you can only eat small amounts at each meal, protein has to be prioritized first, before carbs, before fats, before anything else. Every meal should start with protein.

That sounds simple, and it is, conceptually. But when your appetite is suppressed and food doesn't appeal to you, getting to 100 grams of protein per day requires deliberate planning. Protein shakes become genuinely useful here, not as a replacement for meals, but as a tool for hitting a target that's hard to reach through food alone when your stomach is the size of a tennis ball.

Pair that protein priority with resistance training two to three times per week, and you've addressed the lean mass preservation issue far more effectively than any supplement stack. Your muscles respond to two signals: mechanical load and amino acid availability. Remove either one during caloric restriction and muscle loss accelerates.

What a Smart Supplement Stack Looks Like on GLP-1 Therapy

If a patient sits across from me and asks "just tell me what to take," here's the practical list I walk through. This isn't a one-size-fits-all prescription. It's a starting framework that gets adjusted based on labs and individual needs.

A high-quality multivitamin with methylated B vitamins and chelated minerals covers the broad baseline. Vitamin D3 at 2,000 to 5,000 IU daily (adjusted by lab levels). Magnesium glycinate at 200 to 400mg before bed, which also helps with sleep and muscle cramps. Omega-3 fatty acids (EPA/DHA) at 2 grams daily, since fish consumption often drops with appetite. And a protein supplement if dietary intake consistently falls below target.

What you don't need: expensive "weight loss support" supplements with proprietary blends, green tea extract pills, garcinia cambogia, or any of the dozens of products marketed specifically to people taking GLP-1 medications. The supplement industry has noticed the GLP-1 boom and is producing targeted products at premium prices that contain the same basic nutrients available for a fraction of the cost. Don't fall for it.

The real work here isn't complicated. It's consistent. Take your basic supplements daily, prioritize protein at every meal, get your labs checked on schedule, and communicate with your prescribing physician about symptoms that might signal a gap. Most of the patients I see at Magnolia Functional Wellness in Southlake who run into trouble on GLP-1 therapy aren't doing anything wrong. They just weren't told that the medication that's so good at reducing calories also reduces the nutrients that come with those calories. Now you know, and that puts you ahead of the curve.