People tend to think of supplements as gentle and natural, and of prescription medicines as the strong stuff. In reality, some supplements are powerful enough to change how a medicine works. They can make it weaker, so it stops doing its job, or stronger, so side effects become dangerous. Most interactions are avoidable once you know what to watch for.
Why interactions happen
There are a few common ways a supplement can interfere with a medicine:
- Absorption. Some minerals bind to medicines in the gut and stop them from being absorbed.
- Breakdown. Your liver uses enzymes to clear many drugs from the body. Some herbs speed those enzymes up, which lowers drug levels. Others slow them down, which raises drug levels.
- Doubling up. A supplement may have a similar effect to a medicine, such as thinning the blood or lowering blood sugar, so taking both adds up to too much.
The interactions that come up most often
St. John's wort and many medicines
St. John's wort, an herb taken for low mood, is the best-known troublemaker. It speeds up the breakdown of many drugs, which can make them less effective. That includes some birth control pills, blood thinners, heart medicines, HIV medicines, transplant drugs and some antidepressants. Combining it with prescription antidepressants can also cause a dangerous reaction called serotonin syndrome. If you take any prescription medicine, talk to a pharmacist before trying St. John's wort.
Vitamin K and warfarin
Warfarin, a blood thinner, works by blocking vitamin K. Suddenly taking a supplement with vitamin K, or making big changes to how many leafy greens you eat, can make warfarin less effective. You do not need to avoid vitamin K entirely, but keep your intake steady and tell your doctor about any supplements.
Fish oil, vitamin E, garlic, ginkgo and blood thinners
These can all have mild blood-thinning effects. Combined with prescription blood thinners or daily aspirin, they may raise the risk of bleeding. Many surgeons ask patients to stop them before surgery.
Calcium, iron and magnesium with thyroid medicine and antibiotics
These minerals can bind to levothyroxine, a common thyroid medicine, and to some antibiotics such as tetracyclines and fluoroquinolones, stopping them from being absorbed. The usual advice is to separate them by about four hours from thyroid medicine and by a few hours from those antibiotics. Check the exact timing with your pharmacist.
Potassium and blood pressure medicines
Some blood pressure medicines, such as ACE inhibitors and certain diuretics, raise potassium levels. Adding potassium supplements or salt substitutes made with potassium can push levels too high, which can affect heart rhythm.
Supplements that lower blood sugar
Cinnamon, berberine, chromium and some other products are marketed for blood sugar. If you already take diabetes medicine, adding them could push your blood sugar too low. Talk to your doctor first.
Grapefruit, though not a supplement
Grapefruit and grapefruit juice raise blood levels of some medicines, including certain statins and blood pressure drugs. It is not a supplement, but it is such a common interaction that it is worth knowing.
Melatonin and sedatives
Combining melatonin with sleep medicines, some anxiety medicines or alcohol can increase drowsiness. See melatonin: what to know.
CBD and many medicines
CBD affects the same liver enzymes as many drugs, including some seizure medicines and blood thinners. Read more in our article on whether CBD lives up to its claims.
How to protect yourself
- Keep a complete list of every medicine and supplement you take, with doses. Update it when anything changes.
- Use one pharmacy if you can, so the pharmacist sees everything you take.
- Ask before you start anything new. A two-minute conversation with a pharmacist is free and very effective.
- Read the label for warnings, and look up products on MedlinePlus Herbs and Supplements, which lists known interactions in plain language.
- Watch for changes after starting something new: unusual bruising, a racing heart, dizziness, extreme drowsiness or a medicine seeming to stop working.
Our list of questions to ask your doctor about supplements can help you prepare for that conversation.
Do not stop medicine on your own
If you discover a possible interaction, do not stop a prescription medicine without talking to your doctor. Stopping some medicines suddenly is dangerous. It is usually the supplement that should go, or the timing that should change.
The bottom line
Most supplements are safe for most people, but a handful of common ones can interfere with prescription medicines in important ways. St. John's wort, vitamin K, blood-thinning supplements and minerals taken alongside thyroid medicine are the big ones. Keep a list, use your pharmacist, and ask before you add anything new.