Short answer: usually, yes. Most people taking a daily medicine can also take a multivitamin or a vitamin D tablet with no trouble. But there are a few well-known combinations where a supplement either stops a medicine from being absorbed or changes how strongly it works. Many of these are fixed simply by taking the two at different times.

This post is general information. Your doctor or pharmacist knows your actual medicines, so check with them before starting anything new.

Minerals that block absorption

Calcium, iron, magnesium and zinc can bind to certain medicines in the gut so less of the medicine gets into your blood. The most common examples:

  • Thyroid medicine (levothyroxine). Calcium and iron both reduce how much is absorbed. The usual advice is to take levothyroxine on an empty stomach and keep calcium or iron at least four hours away from it.
  • Some antibiotics. Tetracyclines (such as doxycycline) and quinolones (such as ciprofloxacin) are affected by calcium, iron, magnesium and zinc, including antacids and dairy. A gap of a few hours either side is normally recommended.
  • Bone medicines (bisphosphonates). These have strict rules about taking them with plain water, away from food and supplements.

If you take a calcium and vitamin D tablet, or an iron tablet in pregnancy, ask your pharmacist how to space it from your other medicines. It is a quick question and the answer is usually "morning for one, evening for the other".

Supplements that change how a medicine works

  • Vitamin K and warfarin. Warfarin works against vitamin K. A sudden change in vitamin K intake, from supplements or from a big change in green leafy vegetables, can make it work less well. People on warfarin should keep their intake steady and tell their doctor before starting a vitamin K supplement.
  • Fish oil, garlic, ginkgo and blood thinners. These may add a small extra effect on bleeding. Usually minor, but worth mentioning before surgery or dental work.
  • St John's wort. This herbal product speeds up how the liver breaks down many medicines, including some birth control pills, some heart medicines and some antidepressants. It is one of the few herbs with interactions serious enough that many doctors simply say avoid it.
  • Potassium and some blood pressure medicines. Certain blood pressure tablets already raise potassium, so an extra potassium supplement can push levels too high.
  • Herbs and diabetes medicine. Some plant extracts sold for blood sugar can add to the effect of diabetes tablets or insulin and cause low blood sugar.

Food counts too

Grapefruit and grapefruit juice increase the level of some cholesterol medicines (statins), some blood pressure medicines and others. It is not a supplement, but it is the best-known food interaction and worth knowing about if you take a daily medicine.

Why "natural" is not the same as "safe with everything"

Herbal and Ayurvedic products contain active compounds. That is the point of taking them. The same activity that might help you can also stack with, or work against, a medicine. The problem is that herbal products are rarely mentioned at the doctor's clinic, because people do not think of them as medicine.

A simple routine that avoids most problems

  1. Keep a written list of everything you take: medicines, vitamins, herbal products, and how often.
  2. Show it to your doctor at each visit and to the pharmacist whenever you start something new.
  3. Ask one specific question: "Do I need to take this at a different time from my other medicines?"
  4. Buy supplements from a licensed pharmacy, so you know what is actually in them. Our post on spotting fake or substandard medicine explains what to check.

If you ever take two medicines together and feel unwell, dizzy, unusually drowsy, or notice bruising or bleeding you cannot explain, stop and contact your doctor or pharmacist.

This article is general information, not medical advice. Talk to your doctor or pharmacist about your own medicines and health.