Supplements are medicines in every way that matters for interactions. They contain active compounds, those compounds are processed by the same liver enzymes as prescription medicines, and they can make a medicine stronger, weaker or more dangerous.
The single most useful thing you can do is tell every prescriber and pharmacist what you take, including supplements and herbal products. Supplements are the category people most often leave out, usually because they are thought of as food rather than medicine. A pharmacist can only screen for problems they know about.
Common side effects reported with this supplement or ingredient
An interaction often shows up not as a new symptom but as a familiar one behaving oddly: a medicine that stops working as well, unexpected bruising, or a side effect that is suddenly worse than it used to be. If something changes after you start a supplement, the supplement is a reasonable suspect.
Interactions fall into four patterns:
- Speeding up clearance, so the medicine is removed faster and stops working properly
- Slowing clearance, so the medicine builds up and side effects increase
- Adding to the same effect, such as two things that both thin the blood
- Blocking absorption, where a mineral binds a medicine in the gut
Possible interactions with medicines, chronic conditions or pregnancy
St John’s Wort is the most important single example and interacts with more medicines than any other common supplement. It reduces the effectiveness of hormonal contraception, warfarin, HIV medicines, transplant anti-rejection medicines, some epilepsy and cancer treatments, and others. Combined with antidepressants it can cause serotonin syndrome. Starting or stopping it changes the levels of those medicines, so neither should be done without telling your prescriber. See St John’s Wort interactions.
Supplements that affect bleeding. Fish oil, turmeric, ginkgo, garlic and vitamin E can each add to the effect of warfarin, clopidogrel, aspirin and anti-inflammatories. Individually modest, they add up, and they matter before surgery or dental procedures. See supplements and blood thinners.
Vitamin K and warfarin. The aim here is consistency rather than avoidance. Large swings in vitamin K intake, from food or a supplement, destabilise the INR. See warfarin side effects.
Minerals that block absorption. Calcium, iron, magnesium and zinc bind several medicines in the gut, notably levothyroxine and some antibiotics including the tetracycline and quinolone groups. The fix is usually spacing them several hours apart rather than stopping either. This catches people out because multivitamins contain all four. See levothyroxine side effects.
Potassium. Potassium supplements and low-sodium salt substitutes add to the potassium-raising effect of ACE inhibitors, ARBs and some diuretics. See supplements with blood pressure medication.
Blood sugar. Several supplements marketed for blood sugar can add to diabetes medicines, increasing the risk of hypoglycaemia. See supplements with diabetes medication.
Stimulant supplements. Caffeine-heavy pre-workouts and fat burners add to decongestants, asthma relievers, ADHD medicines and thyroid medicine, producing palpitations and raised blood pressure. See supplements and palpitations.
Sedating supplements. Products for sleep and anxiety, including kava and valerian-type preparations, add to sleeping tablets, antihistamines, opioids and alcohol.
Who should speak to a healthcare professional before using it?
Anyone taking a medicine where the dose is finely balanced or the consequences of failure are serious. That includes:
- Blood thinners, and medicines for heart rhythm
- Epilepsy medicines
- HIV medicines and transplant anti-rejection medicines
- Lithium, methotrexate and other medicines needing blood monitoring
- Hormonal contraception, where reduced effectiveness means unintended pregnancy
- Cancer treatment
Also ask if you are pregnant or breastfeeding, have kidney or liver disease, or have surgery planned. See supplements before surgery.
Dose, product-quality and stacking risks
Supplements in South Africa are regulated as complementary medicines, and the checks applied are not the same as those for prescription medicines. Strength and content can vary between products, and herbal preparations vary particularly widely, which means the size of an interaction is not always predictable from the label. See how supplements are regulated in South Africa.
Combination products complicate this further, since a single sleep or immune formula may contain several actives, any of which could interact. Read the full ingredient panel rather than the product name. See taking too many supplements.
When to stop and get medical advice
Get urgent help for: unusual bruising or bleeding that will not stop; agitation or confusion with fever, sweating and a racing heart; or a suspected clot, with pain and swelling in one leg or sudden breathlessness.
Contact a doctor or pharmacist promptly if a condition being treated seems to be worsening after starting a supplement, a monitored blood test moves unexpectedly, or you have been taking a supplement alongside a prescription medicine without their knowledge.
Do not stop a prescribed medicine because you suspect an interaction. Stop the supplement and ask.
Related supplement safety guides
- Supplement side effects
- Supplements with blood thinners
- Supplements before surgery
- When to see a doctor about a supplement
- Report a supplement side effect in South Africa
See also supplement and medication interactions and chronic medication and supplements.
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.