6 Medications You Shouldn’t Mix with Your Multivitamin

About one-third of U.S. adults take a daily multivitamin, often to fill nutritional gaps or as a form of “insurance.” While most multivitamins are safe for healthy adults, combining them with certain medications can change how your body absorbs either the supplement or the drug itself. That’s because vitamins and minerals share the same absorption pathways, compete for enzymes and can alter how quickly medications are metabolized. As a result, drug-nutrient interactions may change how your medication works.

If you regularly take prescription or over-the-counter medications, it’s worth reviewing whether your multivitamin could influence their effects. Below, experts highlight common drugs that deserve extra attention and explain how to time your doses to avoid interactions.

Thyroid Medications (Levothyroxine)

Thyroid medication (levothyroxine) is one of the most common prescriptions affected by multivitamins. Calcium and iron can bind to levothyroxine in the digestive tract, forming complexes that block absorption and make the drug less effective. 

To avoid this, take levothyroxine first thing in the morning on an empty stomach with water, explains Serena Pratt, M.S., RD. Then wait at least four hours before taking any supplement containing calcium, iron or magnesium to ensure that the medication is properly absorbed.

Blood Thinners (Warfarin)

Warfarin is a medication used to prevent blood clots by blocking vitamin K, a nutrient your body needs for normal clotting. Because many multivitamins contain small amounts of vitamin K, suddenly starting or stopping one can interfere with warfarin’s effects.

People taking warfarin don’t have to avoid multivitamins completely, but consistency is key. Pratt explains that fluctuating vitamin K intake can make warfarin less effective or increase bleeding risk.  Easy bruising, unusual bleeding or new leg swelling may signal a clotting change, especially if vitamin K intake from supplements has shifted, she adds.

Antibiotics (Tetracyclines and Fluoroquinolones)

Some antibiotics, such as doxycycline or ciprofloxacin, can bind to minerals found in multivitamins—including magnesium, calcium, iron and zinc—which can decrease the antibiotic’s effectiveness.,, If taken at the same time as a multivitamin, the antibiotic may not reach therapeutic levels, explains Erica Park, M.D., M.B.A., FAAPMR.

To avoid this, separate your antibiotic and multivitamin doses by at least two hours. If you’re prescribed multiple daily antibiotic doses, your pharmacist can help you create a schedule that minimizes overlap.

Osteoporosis Medications (Bisphosphonates)

Bisphosphonates, such as alendronate (Fosamax), are highly sensitive to absorption interference. They must be taken on an empty stomach with plain water, and even small amounts of calcium, iron or other minerals from a multivitamin can reduce their effectiveness., 

“[These medications] work best when taken alone [with] no food, no coffee and definitely no supplements at the same time,” explains Park. It’s generally recommended to wait at least two hours before taking a multivitamin after taking a bisphosphate.

Diuretics 

Diuretics, often used for blood pressure, affect how your kidneys handle important electrolytes like calcium, magnesium and potassium. Taking a multivitamin that also contains these minerals may alter their absorption and metabolism.,

Thiazide diuretics tend to increase calcium retention by reducing the amount excreted through urine, while loop diuretics can lead to significant losses of magnesium, potassium and calcium.,

When taken with thiazide diuretics, high calcium intake from supplements or multivitamins may raise blood calcium, which in rare cases can cause nausea, confusion or irregular heartbeat, says Park. For anyone on diuretics, health care providers often recommend checking electrolyte levels with a lab test before adding supplements to avoid potential interactions.

Antacids and Proton Pump Inhibitors (PPIs)

Long-term use of acid-reducing medications—like proton pump inhibitors (PPIs) such as omeprazole, or some antacids—can reduce the absorption of certain nutrients, including vitamin B12, magnesium and iron. Stomach acid helps release these nutrients from food and supplements, so lowering acidity may impair absorption over time. Because PPIs continually suppress stomach acid, it’s best to discuss with your health care provider alternate strategies, such as sublingual vitamin B12, injections or nutrient formulations that don’t rely on stomach acid for absorption. For antacids, Pratt recommends taking your multivitamin at a different time.

Our Expert Take

Experts agree that multivitamins can be useful—but they’re meant to complement, not replace, a balanced diet. “[Multivitamins] supplement nutrients that may be lacking in the diet, not to replace real food,” says Pratt. A multivitamin can help fill nutritional gaps, but personalization is key, adds Park.

The safest approach is to focus on nutrient-rich foods like fruits, vegetables, whole grains, legumes, healthy fats and lean proteins, and use supplements only when indicated by your health care provider. If you take prescription medications, bringing your supplement list to every appointment can help your health care team screen for possible interactions.