Guide
Supplements and common medicines: interactions to know about
"Natural" does not mean "no side effects". Many supplements contain active ingredients that can strengthen, weaken or otherwise change the effect of a prescription or over-the-counter medicine. Most interactions are avoidable once you know where to look. This guide covers well-documented examples. It is general information, not a substitute for advice about your own medicines, so always check with your doctor or pharmacist.
Why interactions happen
There are a few common ways a supplement can interfere with a medicine:
- Doing the same job. If a supplement and a medicine both affect blood clotting, taking them together can add up.
- Working against it. A nutrient can directly counter what a medicine is meant to do.
- Changing how the body processes it. Some herbs speed up the enzymes that clear medicines from the body, so less of the medicine stays active.
- Blocking absorption. Minerals can bind to some medicines in the gut, so less of the medicine gets into the body.
Blood thinners such as warfarin
Blood thinners are the classic example because the margin for error is narrow.
The FDA warns that warfarin, ginkgo biloba, aspirin and vitamin E can each thin the blood, and that "taking any of these products together may increase the potential for internal bleeding or stroke."
Vitamin K works the other way: it helps blood clot, which is the process warfarin slows down. MedlinePlus advises people on warfarin to "eat consistent amounts of vitamin K-containing food on a week-to-week basis" and not to eat large amounts of leafy green vegetables or certain vegetable oils high in vitamin K. A supplement that adds vitamin K, including some bone and multivitamin formulas with vitamin K2, deserves the same care.
MedlinePlus also lists several supplements that may interact with warfarin: coenzyme Q10, echinacea, garlic, ginkgo biloba, ginseng, goldenseal and St. John's wort. Its advice: do not start any of these without discussing it with your health care provider.
In our quiz, products with ingredients known to affect blood thinners are marked as blood-thinner sensitive, and if you tell us you take a blood thinner, we add a note to check with your doctor.
Thyroid medicine (levothyroxine)
Levothyroxine is usually taken on an empty stomach. MedlinePlus says it "usually is taken once a day on an empty stomach, 30 minutes to 1 hour before breakfast."
Some minerals can reduce how much is absorbed. MedlinePlus advises taking calcium carbonate or iron "at least 4 hours before or 4 hours after you take levothyroxine." Many multivitamins, bone formulas and mineral supplements contain calcium or iron, so check the label and plan your timing with your pharmacist.
Antidepressants, birth control and St. John's wort
St. John's wort is one of the best-studied examples of an herb that changes how medicines work. According to the FDA, drugs for HIV/AIDS, heart disease, depression and organ transplants, as well as birth control pills, can be less effective when taken with St. John's wort.
NCCIH adds that combining St. John's wort with certain antidepressants can lead to serotonin syndrome, which ranges from tremor and diarrhea to dangerous confusion, muscle stiffness and drops in body temperature. Some menopause and mood blends include St. John's wort, so read the ingredient list even when the product is sold for a different purpose.
Antibiotics, osteoporosis medicine and minerals
The NIH Office of Dietary Supplements notes that magnesium may interact with oral bisphosphonates (a type of osteoporosis medicine) and with tetracycline and quinolone antibiotics. Separately, some medicines, including diuretics and proton pump inhibitors used for acid reflux, can affect magnesium levels.
The practical step is usually spacing: taking the mineral several hours apart from the medicine. The exact timing depends on the medicine, so ask your pharmacist.
Before surgery
Some supplements can increase bleeding or affect anesthesia. NCCIH notes that "certain dietary supplements may increase the risk of bleeding or affect your response to anesthesia." The FDA adds that your health care professional "may ask you to stop taking dietary supplements two or three weeks before the procedure." Tell your surgical team about everything you take, as early as you can.
Pregnancy and breastfeeding
NCCIH points out that many supplements "have not been tested in pregnant women, nursing mothers, or children." Some herbs commonly found in hormone, menopause and stress blends carry label warnings against use in pregnancy. If you are pregnant, trying to conceive or breastfeeding, talk to your doctor or midwife before starting anything new.
How to stay safe
The FDA's advice is practical:
- Keep a list of every medicine and supplement you take, with doses and how often. Bring it, or the bottles, to every appointment.
- Ask before you add. Call your doctor or pharmacist before starting a new supplement, and mention everything else you take.
- Mention changes, such as pregnancy, a new diagnosis, recent illness or surgery.
- Read the whole label, including blends. An interaction can hide in a product sold for something unrelated.
- Stop and call if you notice side effects. NCCIH's advice is to stop taking the supplement and contact your health care provider.
Pharmacists are a good resource here: they can check your full list of medicines and supplements and tell you how far apart to take things.
How our quiz helps
When you take our quiz, the "Quick safety check" step asks whether you take blood thinners, thyroid medication, or antibiotics, statins or antacids, and whether you are pregnant or nursing. We use your answers only on your device, to add "check with your doctor" notes to products with known interactions. That is a prompt for a conversation, not medical advice, and it cannot know about every medicine. Your doctor and pharmacist are the right people to make the final call.