With age and new (or more) chronic health conditions, it’s common for people to need prescription medications. Many people also use over-the-counter products and supplements to meet their health needs.1

Although the use of dietary supplements and medications is common, particularly among older adults, experts say that taking certain medicines and supplements at the same time could have dangerous—even life-threatening—consequences.2

The National Center for Complementary and Integrative Health (NCCIH) says that many people combine supplements and prescription or over-the-counter (OTC) medications without being aware of the potential interactions.3

“Combining certain dietary supplements and medications can lead to a variety of harmful effects, including diminished drug effectiveness, increased risk of drug toxicity, and unexpected side effects,” Danielle Crumble Smith, RDN, a certified registered dietitian at Top Nutrition Coaching, told Verywell.

If you take any medications and supplements, here are the potentially risky combinations that experts say you should be aware of and avoid.

Supplement and Medication Combinations That Can Reduce Effectiveness

Marilyn Tan, MD, double board certified in endocrinology and internal medicine and a clinical associate professor of medicine at Stanford University School of Medicine, told Verywell that some supplements and vitamins can affect the absorption or metabolism of prescription drugs. There are others, like vitamin K, that can even counter the effects of certain medications.

Here are a few possible interactions to be aware of:

• Vitamin B6 and levodopa. Taking vitamin B6 with levodopa, a medication for Parkinson’s disease, can reduce the medication’s effectiveness. However, some levodopa medications also contain carbidopa, which can counteract the interaction.4 People using levodopa or levodopa-carbidopa intestinal gel (LCIG) can develop a vitamin B6 deficiency. This can cause anemia, depression, neuropathy, and other issues. A healthcare provider may monitor you and advise B6 supplementation. It’s OK to take vitamin B6 if you’re on LCIG under your healthcare provider’s advisement.
• Vitamin K and blood-thinning medications. Vitamin K can counteract the effects of blood-thinning medications like warfarin, possibly making them less effective. If you’re on blood-thinning medications, talk with your provider before taking vitamin K supplements or increasing your dietary intake of vitamin K.5
• Iron or calcium and thyroid hormone replacement medications. Certain supplements like iron and calcium can interfere with the absorption of thyroid hormone replacement medications like levothyroxine. It’s best to take these supplements and thyroid hormone replacement medications at least four hours apart.6

Potentially Harmful Supplement and Medication Combinations

According to Crumble Smith, there are also combinations of supplements and medications that can cause harmful side effects or symptoms:

• Vitamin C and antacids that contain aluminum. Vitamin C increases the absorption of aluminum from antacids—medicines like Tums) that neutralize stomach acid and help relieve heartburn or indigestion.7 However, taking vitamin C and antacids together can lead to toxic levels of aluminum in the body. Therefore, they should be taken at least two hours apart.8
• Vitamin E and blood-thinning medications. Vitamin E can increase the risk of bleeding when it’s taken with blood-thinning medications like warfarin. If you are taking a blood thinner, ask your provider before you start taking vitamin E.9
• St. John’s Wort and various medications. St. John’s Wort supplements can interact with many medications, including antidepressants, birth control pills, and blood thinners. St. John’s Wort can reduce the effectiveness of these medications, and mixing them can cause harmful effects.10
• Goldenseal and blood clotting medications. Goldenseal supplements can interact with some medications, including clotting medications. The interaction may reduce the body’s ability to clot blood, which can lead to bleeding or bruising. Given the seriousness of the interaction, it’s often recommended to avoid using goldenseal and blood clotting medications together.11
• Ginkgo biloba and blood-thinning medication. Ginkgo biloba supplements can also increase the risk of bleeding when taken with blood-thinning medications like warfarin or aspirin.12
• Licorice root and diuretics or blood pressure medications. Supplements that contain licorice root can interact with diuretics and blood pressure medications, making their side effects worse, or even causing serious problems like potassium imbalances or blood pressure fluctuations in the body.13

If you’re taking a prescription medication and thinking about trying a supplement, talk to your provider or pharmacist.

What to Do If You Take These Combinations

If you are prescribed medications and plan on using supplements, timing is everything. For example, Tan said that thyroid hormone medications should be taken on an empty stomach and separated from supplements by at least four hours—especially if they contain calcium and iron.

“Other endocrine medications like the osteoporosis medication Fosamax and the diabetes medication Rybelsus need to be taken on an empty stomach and separated from other medications by at least 30 minutes,” Tan added.

To avoid potential interactions, Crumble Smith recommends talking about the specifics of timing your doses with your provider. As a general rule of thumb, space out when you take supplements and your medications by at least two hours.

When to Check With Your Healthcare Provider 

Your provider can guide you on what combinations to avoid and how to safely use supplements with medications.

In general, it’s never a bad idea to let your provider know if you’re thinking about trying something new since you won’t necessarily know about the possible risks involved.

“Supplements seem harmless, but it really is important to consult a healthcare professional if you are considering starting a new supplement,” Crumble Smith said. “This is particularly true if you are currently taking medications.”

How to Check for Interactions Yourself

You can also ask your pharmacist about drug-supplement combinations when you’re picking up your medications. Pharmacists are trained to know about drug interactions, including those that can happen with dietary supplements. They also have access to comprehensive databases and tools that let them check for potential interactions.

A pharmacist can give you advice on how to use supplements safely, check for interactions, make suggestions about the timing of your doses, or offer alternatives. In some cases, a pharmacist might be able to suggest dose adjustments that could help you avoid an interaction.

Crumble Smith said that it’s important to give a pharmacist a complete list of everything you’re taking—from prescription meds to OTC products and supplements—as this information will help them make the most accurate assessment.

There are also several databases and online tools from reputable resources that you can use to check for potential drug and supplement interactions on your own, including:

• MedlinePlus: This website is sponsored by the National Institutes of Health (NIH) and managed by the U.S. National Library of Medicine. It offers a lot of information about medications, including potential interactions between them. 
• National Center for Complementary and Integrative Health (NCCIH): The NCCIH website offers general advice on the safe use of supplements and gives overviews of specific herbs and supplements. 
• Drugs.com Interaction Checker: This website is a comprehensive resource for checking drug, herbs, and supplement interactions. You can enter the names of the drugs and/or supplements that you're taking, and it will show you any known interactions.

Crumble Smith said that while these resources can provide useful information, they are not a substitute for professional medical advice.

“Always consult with a healthcare provider or pharmacist before starting, stopping, or modifying your medication or supplement regimen,” she said.