Vitamins and Supplements to Take With GLP-1 Medications

*Compounded semaglutide and [compounded tirzepatide](/resources/compounded-tirzepatide-guide) are not FDA-approved medications. This article is for informational purposes only and does not constitute medical advice. Consult your licensed healthcare provider before starting any supplement or medication. Care at Prescriva is delivered by independently licensed providers, not by Prescriva LLC, doing business as Prescriva, which is a management services organization.*

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One thing nobody warns you about before starting a GLP-1 medication: you are going to eat a lot less. That is the point. GLP-1 receptor agonists like semaglutide and tirzepatide slow gastric emptying and suppress appetite, and most people on these medications naturally reduce their caloric intake by 20 to 40 percent.

Less food means less of everything: fewer calories, fewer grams of protein, and meaningfully fewer vitamins and minerals. For people already at nutritional risk before starting treatment, that reduction matters.

This is not a reason to avoid GLP-1 medications. It is a reason to think carefully about nutritional support while you are on them. Several specific nutrients deserve attention during treatment, and a 2026 review in *Nutrients* describes this as "the nutritional paradox of obesity" - people with excess body weight are simultaneously at high risk for micronutrient deficiencies, and weight loss interventions can deepen that gap if nutrition is not actively managed. [1]

Here is what the research shows about the key vitamins and supplements worth discussing with your provider during GLP-1 treatment.

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Why Nutrient Deficiencies Are Common in Obesity (and on GLP-1 Therapy)

Before we get to specific supplements, it helps to understand the baseline. People with obesity are disproportionately likely to have low levels of vitamin D, magnesium, and B vitamins relative to people at normal body weight. This is not a lifestyle failure. It is a biological reality.

Fat-soluble vitamins (like vitamin D) are sequestered in adipose tissue, making them less bioavailable in the bloodstream. High-calorie diets often lack micronutrient density. Chronic low-grade inflammation associated with excess adiposity impairs nutrient metabolism.

When you add a medication that meaningfully reduces food intake, those deficiencies do not automatically correct themselves. In some cases, they worsen. The goal of supplement support during GLP-1 treatment is to fill those gaps so that your body can function optimally as you lose weight.

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Vitamin D

Vitamin D deficiency is remarkably common in people with obesity. A 2021 review in the *International Journal of Molecular Sciences* found that individuals with obesity have substantially lower circulating 25-hydroxyvitamin D levels compared to people with normal body weight, driven by fat-tissue sequestration, reduced outdoor activity, and dietary patterns that often lack fortified or fatty foods. [2]

This matters for GLP-1 treatment for two reasons. First, vitamin D plays a role in insulin sensitivity and glucose metabolism. A 2023 review in *Nutrients* found that low vitamin D status is associated with increased insulin resistance and impaired beta-cell function, and supplementation in deficient individuals may support improvements in glycemic markers. [3] Second, people eating less food naturally consume less vitamin D from dietary sources.

What the research suggests: Getting your 25-hydroxyvitamin D level tested before or early in GLP-1 treatment is a reasonable first step. Many clinicians treating patients in medically supervised weight loss programs routinely include this panel. If you are deficient, supplementation with 1,000 to 4,000 IU daily (based on your level and provider guidance) is a common approach.

A 2022 study in *Scientific Reports* found that vitamin D deficiency in adults with obesity correlated with elevated inflammatory markers and vascular dysfunction, suggesting that correction of deficiency may have benefits beyond bone health during a period of significant metabolic change. [4]

*Talk to your provider about testing your vitamin D level and whether supplementation is appropriate for you.*

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Vitamin B12

Vitamin B12 deserves specific attention for anyone on GLP-1 treatment who is also taking metformin, a medication frequently co-prescribed in patients with type 2 diabetes or insulin resistance.

Metformin is known to reduce B12 absorption by interfering with the intrinsic factor pathway in the gut. A 2026 study in the *Romanian Journal of Diabetes, Nutrition and Metabolic Diseases* found significant correlations between metformin use, elevated homocysteine, and reduced B12 levels in type 2 diabetes patients, underscoring the importance of monitoring this nutrient during long-term therapy. [5]

Even for people not on metformin, reduced food intake during GLP-1 treatment means less dietary B12. Animal proteins - meat, fish, eggs, dairy - are the primary sources of B12, and when appetite suppression leads to eating less of these foods, intake drops.

B12 deficiency develops slowly, often over months, and its early symptoms (fatigue, numbness, brain fog) overlap with common side effects of GLP-1 medications, making it easy to miss. Monitoring B12 levels periodically during treatment is worth discussing with your provider, particularly in the first 6 to 12 months.

Methylcobalamin vs. cyanocobalamin: Both are effective forms of B12 in supplement form. The standard oral dose for maintenance is 500 to 1,000 mcg daily. Sublingual forms are sometimes preferred for absorption.

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Magnesium

Magnesium is one of the most underappreciated minerals in metabolic health, and it is consistently depleted in people with obesity and insulin resistance.

A 2025 meta-analysis in *Biological Trace Element Research* found that children and adolescents with overweight or obesity had significantly lower serum magnesium levels than normal-weight peers, and that magnesium deficiency was associated with elevated insulin resistance markers. [6] A 2022 review in *International Journal of Molecular Sciences* describes magnesium's role in over 300 enzymatic reactions, including those governing glucose uptake, insulin signaling, and ATP synthesis, making deficiency a potential drag on metabolic function. [7]

For people on GLP-1 medications, magnesium has an additional practical benefit: it supports bowel regularity. Constipation is a common side effect of GLP-1 therapy, caused by slowed gastric motility and reduced food volume moving through the digestive tract. Magnesium (particularly magnesium citrate or oxide) draws water into the colon and supports regular bowel movements without harsh stimulant laxatives.

What to know about supplementing magnesium:

• Magnesium glycinate is the gentlest form for daily use and is well-absorbed without causing loose stools at moderate doses
• Magnesium citrate has a mild laxative effect and works well for constipation management
• Magnesium oxide is the most affordable form but the least bioavailable
• Typical doses range from 200 to 400 mg daily for adults
• People with kidney disease should consult their provider before supplementing magnesium

Omega-3 Fatty Acids

Omega-3 fatty acids (EPA and DHA, primarily from fish oil) have a well-established evidence base for supporting cardiovascular health and reducing inflammation. In the context of GLP-1 treatment, both matter.

GLP-1 medications like semaglutide already demonstrate cardiovascular benefits (the SELECT trial showed a 20% reduction in major adverse cardiovascular events in people with obesity and established cardiovascular disease). Omega-3 supplementation represents a complementary, non-pharmacological strategy for people looking to support heart health during weight loss treatment.

A 2023 systematic review and meta-analysis in *Nutrients* examined the effect of alpha-linolenic acid (ALA) supplementation in individuals with overweight or obesity and found improvements in total cholesterol and LDL levels, suggesting cardiovascular risk profile benefits in this population. [8]

For triglycerides specifically - which are elevated in many people with metabolic syndrome - high-dose EPA/DHA supplementation has a robust evidence base. Prescription-strength omega-3 therapies (like icosapent ethyl) are FDA-approved for severe hypertriglyceridemia, while standard OTC fish oil supplements at 1 to 2 grams daily provide meaningful support at lower doses.

What to look for in a fish oil supplement:

• A combined EPA + DHA content of at least 500 mg per capsule
• Third-party testing certification (IFOS or NSF International) to verify purity and absence of heavy metals
• Triglyceride form (rather than ethyl ester form) for better absorption
• Storage in a cool, dark place to prevent oxidation

What to Discuss With Your Provider

Before starting any supplement during GLP-1 treatment, a conversation with your prescribing provider is worth having. This is not about caution for its own sake. It is about making sure supplements fit your specific situation.

A few things worth raising:

• Lab testing: Baseline vitamin D, B12, and magnesium levels help you supplement based on actual need rather than guessing
• Timing: Some supplements (calcium, for example) can interfere with the absorption of thyroid medication or other drugs - spacing them out from other medications matters
• Dose: Fat-soluble vitamins (A, D, E, K) accumulate in body fat and can reach toxic levels with excessive supplementation. Water-soluble vitamins (like B12 and C) are generally forgiving, but dose still matters
• GI tolerance: During the adjustment period on GLP-1 medications, when nausea and stomach sensitivity are most common, large capsule supplements can be harder to tolerate. Starting with smaller doses and gradually increasing, or splitting doses with meals, can help

Supplements to Approach Carefully

Not every supplement marketed for weight loss or metabolic health is backed by solid evidence or appropriate for people on GLP-1 therapy.

A few worth approaching with caution:

• High-dose vitamin A: Fat-soluble vitamins accumulate in adipose tissue, and excess vitamin A can cause liver toxicity over time. A standard daily multivitamin provides adequate vitamin A for most people
• Herbal weight loss supplements: Many are unregulated, lack clinical evidence, and some interact with medications. Stimulant-containing supplements (caffeine, synephrine, yohimbine) can raise heart rate and blood pressure
• Exogenous ketones or ketone salts: Not harmful in most people, but evidence for weight loss benefit is limited, and they add cost without clear value during GLP-1 treatment
• High-dose zinc: Zinc is an important mineral, but doses above 40 mg daily can interfere with copper absorption and create deficiency over time. A standard multivitamin dose is sufficient for most people

A Practical Starting Point

If you are starting or currently on a GLP-1 medication and want to support your nutritional status, the most practical approach is to ask your provider to include a basic micronutrient panel at your next visit: vitamin D (25-OH), vitamin B12, magnesium, and a standard CBC to check for anemia. These are inexpensive labs that provide a meaningful baseline.

From there, targeted supplementation based on your actual levels is more useful than a generic high-dose multivitamin regimen. Many people find that a high-quality daily multivitamin covers the basics, with targeted additions for specific deficiencies where needed.

GLP-1 medications do a lot of the metabolic heavy lifting. Supporting them with solid nutritional foundations keeps your body in the best position to benefit from the changes underway.

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Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult a licensed healthcare provider before starting any supplement or medication.

Compounding Disclaimer: Compounded semaglutide and compounded tirzepatide are not FDA-approved medications. Compounded drugs are not reviewed by the FDA for safety, efficacy, or quality. Compounded semaglutide is not the same as, equivalent to, or interchangeable with FDA-approved semaglutide products (Ozempic, Wegovy, or Rybelsus).

Results Disclaimer: Individual results vary. Weight management outcomes depend on adherence to your prescribed treatment plan, diet, exercise, starting weight, and other individual health factors. Results are not guaranteed.

Provider Disclaimer: All medical services, including prescribing, are provided by independently licensed healthcare providers. Prescriva LLC, doing business as Prescriva is a management services organization and does not practice medicine or make clinical decisions.

Brand Disclaimer: Ozempic and Wegovy are registered trademarks of Novo Nordisk A/S. Mounjaro and Zepbound are registered trademarks of Eli Lilly and Company. Prescriva is not affiliated with, endorsed by, or sponsored by these companies.

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References

1. Alexa RE, et al. The Nutritional Paradox of Obesity: Mechanisms and Clinical Implications of Micronutrient Deficiencies. *Nutrients.* 2026. PMID: 42029584

1. Karampela I, et al. Vitamin D and Obesity: Current Evidence and Controversies. *Int J Mol Sci.* 2021;22(19):10412. PMID: 33792853

1. Argano C, et al. The Role of Vitamin D and Its Molecular Bases in Insulin Resistance, Diabetes, Metabolic Syndrome, and Cardiovascular Disease: State of the Art. *Nutrients.* 2023;15(8):1845. PMID: 37895163

1. Mirza I, et al. Obesity-Associated Vitamin D Deficiency Correlates with Adipose Tissue DNA Hypomethylation, Inflammation, and Vascular Dysfunction. *Sci Rep.* 2022. PMID: 36430854

1. Dundua K, et al. Correlations Between Homocysteine and Vitamin B12 in Type 2 Diabetes Treated With Metformin. *Rom J Diabetes Nutr Metab Dis.* 2026. PMID: 42107930

1. Mashayekhi Y, et al. Role of Serum Magnesium Deficiency in Insulin Resistance Among Overweight and Obese Children: A Meta-Analysis. *Biol Trace Elem Res.* 2025. PMID: 40895688

1. Pelczyńska M, et al. The Role of Magnesium in the Pathogenesis of Metabolic Disorders. *Int J Mol Sci.* 2022;23(9):5126. PMID: 35565682

1. Yin S, et al. Effect of Alpha-Linolenic Acid Supplementation on Cardiovascular Disease Risk Profile in Individuals with Obesity or Overweight: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. *Nutrients.* 2023;15(19):4250. PMID: 37778442