Which supplements interact with common medicines in South Africa?
Supplements that most often interact with common South African medicines include St John's wort, vitamin K and high-dose fish oil with warfarin, red yeast rice with statins, potassium with blood-pressure tablets, and calcium, iron or magnesium with thyroid or antibiotic medicines. Ask a pharmacist to review every product you take, including herbal ones.
- Vitamin K, St John's wort and high-dose fish oil are the supplements most often flagged with warfarin.
- Red yeast rice contains a lovastatin-like compound and can duplicate statin therapy.
- Potassium supplements and salt substitutes can raise potassium dangerously with ACE inhibitors, ARBs and spironolactone.
- Separate calcium, iron and magnesium from levothyroxine, tetracycline and quinolone antibiotics, and bisphosphonates by a few hours.
- Unregistered complementary medicines in South Africa carry a SAHPRA disclaimer; it is not a safety endorsement.
Which supplements interact with common medicines?
Interactions are more likely in older adults because they take more medicines and have reduced kidney and liver clearance. Supplements are not always harmless extras: some change how drugs are absorbed, processed or how strongly they act. Unregistered complementary medicines sold in South Africa must carry a SAHPRA disclaimer saying they have not been evaluated, so a label does not prove safety.
Which combinations matter most?
| Medicine | Supplement | Concern |
|---|---|---|
| Warfarin | Vitamin K (including D3+K2 products and some multivitamins) | changes anticoagulant effect |
| Warfarin, aspirin, other blood thinners | High-dose fish oil, ginkgo, vitamin E | extra bleeding tendency |
| Warfarin, many others | St John's wort | induces enzymes and can reduce drug levels |
| Statins | Red yeast rice | contains a lovastatin-like compound, so doubling up |
| ACE inhibitors, ARBs, spironolactone | Potassium supplements, salt substitutes | raised blood potassium |
| Thiazide diuretics | Calcium and vitamin D | raised blood calcium |
| Levothyroxine | Calcium, iron, magnesium | poorer absorption if taken together |
| Tetracycline or quinolone antibiotics, bisphosphonates | Magnesium, iron, calcium, zinc | poorer absorption |
| Metformin, acid-reducing medicines | Supplements are not the issue | long-term use can lower B12 (and magnesium with acid reducers) |
What practical rules help?
- Space doses: keep calcium, iron and magnesium a few hours away from thyroid, antibiotic and bisphosphonate medicines, as your pharmacist advises.
- Do not self-start St John's wort if you take any regular medicine.
- Check "electrolyte" and "salt" products if you have high blood pressure, heart or kidney disease; many contain sodium or potassium.
- Warfarin patients: tell your clinic about any vitamin K, fish oil, herbal or multivitamin product and keep intake steady.
- Kidney disease: magnesium supplements and creatine need medical sign-off. See supplements and kidney disease.
What about the supplements people ask about most?
For omega-3, the 2025 ISSN position stand notes that high doses may increase bleeding tendency with anticoagulants. For vitamin D, the issues are calcium with thiazides and, with K2 combinations, warfarin. See vitamin D and B12 over 60, CoQ10 and glucosamine and chondroitin for specific product notes.
How do you check in South Africa?
- List everything you take: prescription, over-the-counter, vitamins, herbs.
- Take the packs to a Dis-Chem or Clicks pharmacist or your clinic for a medicines review; this is usually free to ask.
- Include doses and the time of day you take each one.
- Update the list whenever something changes.
- Report suspected side effects to your doctor or pharmacist (see how to report a supplement side effect).
When should you see a doctor?
See a doctor urgently for unusual bruising or bleeding, black stools, palpitations, new muscle weakness, severe diarrhoea or confusion after starting a supplement. This is general information, not personal medical advice.
Related questions
- Which supplements are unsafe with kidney disease in South Africa?
- Do people over 60 need vitamin D and B12 supplements in South Africa?
- Does CoQ10 work for heart or energy?
- Do glucosamine and chondroitin work?
- Does omega-3 help with ageing?
- How do you report a supplement side effect in South Africa?
Sources
- NHS Specialist Pharmacy Service: Using contraception with enzyme-inducing medicines (St John's wort as an inducer)
- Jäger et al. 2025, ISSN Position Stand: long-chain omega-3 polyunsaturated fatty acids (JISSN)
- SAHPRA Guideline 7.04: Complementary Medicines, Health Supplements safety and efficacy (Mar 2022)
Educational information only, not medical advice. Speak to a doctor, pharmacist or registered dietitian before using supplements if you are pregnant or breastfeeding, under 18, taking medication, or living with kidney, liver or heart disease.
Cite this page: Healthnews24, “Which supplements interact with common medicines in South Africa?”, updated 27 October 2026, https://www.healthnews24.co.za/answers/supplement-drug-interactions-for-older-adults-south-africa/. Plain-text version: /answers/supplement-drug-interactions-for-older-adults-south-africa.md