Medicine interactions – the combinations worth knowing
Interactions are why one pharmacy should see your whole list. The ones that matter most in South Africa, and how to avoid them without a pharmacology degree.
How interactions actually cause harm
Two medicines can interact in three broad ways: one makes the other stronger, one makes the other weaker, or the two add up to an effect neither would cause alone. The dangerous version is usually the third — two things that each cause drowsiness, or each thin the blood, or each strain the kidneys.
The practical protection is not memorising a list. It is making sure one person sees everything you take.
The combinations that matter most here
- Two products containing paracetamol. The most common accidental overdose in South Africa, because paracetamol hides inside flu sachets and combination painkillers.
- Anti-inflammatory painkillers with blood pressure tablets, water tablets or kidney disease — a well-recognised route to acute kidney injury, especially when dehydrated.
- Anti-inflammatories with blood thinners or with steroids, which raises the risk of stomach bleeding substantially.
- Sedatives, sleeping tablets, strong painkillers and alcohol in any combination — the effects add up and suppress breathing.
- TB and HIV medicines with a long list of others. Rifampicin in particular speeds up the breakdown of many drugs, including hormonal contraception, which then fails.
- Erectile dysfunction tablets with nitrate medicines for angina — a potentially fatal drop in blood pressure.
- Grapefruit juice with several statins and blood pressure medicines, which raises drug levels.
- Warfarin with almost anything, including antibiotics and some herbal products.
Medicines that most often need an interaction check:
Warfarin — Cipla-Warfarin, Marevan, Coumadin, Warfant
Prevention and treatment of VTE; atrial fibrillation; mechanical heart valves
Rifampicin — Rimactane, Rifadin
Tuberculosis (in combination); leprosy; prophylaxis of meningococcal disease
Amiodarone — Cordarone, Aratac
Ventricular and supraventricular arrhythmias; atrial fibrillation
Lithium Carbonate — Camcolit, Quilonum
Bipolar disorder (prophylaxis and acute mania); recurrent depression augmentation
Methotrexate — Abitrexate, Methoblastin
Rheumatoid arthritis; psoriasis; psoriatic arthritis; ectopic pregnancy
Phenytoin — Epanutin, Dilantin
Epilepsy (focal and generalised tonic-clonic); status epilepticus (IV)
Herbal and traditional products are not exempt
The most consequential interactions people fail to mention are the ones they do not think of as medicine.
- St John's wort reduces the effectiveness of HIV medicines, hormonal contraception, warfarin and several antidepressants — and can cause a dangerous reaction with the latter.
- Grapefruit, in juice or fruit form, affects a surprising number of medicines.
- High-dose vitamin K affects warfarin; so does a sudden change in how many green vegetables you eat.
- Traditional preparations may contain active compounds nobody has characterised, and some are directly toxic to the liver or kidneys.
Protecting yourself in practice
- Keep one written list of everything: prescriptions, pharmacy purchases, vitamins, herbal and traditional remedies. Take it to every appointment.
- Use one pharmacy where you can, so a single system sees the whole picture.
- Ask the pharmacist directly whenever anything is added — this is exactly what they are for and it takes a minute.
- Read the ingredients of anything bought over the counter, especially cold and flu products.
- Ask about alcohol specifically when a new medicine is started.
- Tell every prescriber about HIV and TB treatment. This is the interaction set most often missed, and the consequences are the most serious.
Common questions
- Is it safe to take paracetamol and an anti-inflammatory together?
- They are different classes and are often used together or alternated. The risk is not the combination itself but accidentally doubling up on either through a combination product. Check the ingredients of everything you take in a day.
- Do I need to mention vitamins?
- Yes. High-dose vitamin K, vitamin E, iron, calcium and several herbal supplements all interact with common medicines. Iron and calcium in particular block absorption of thyroid medicine and some antibiotics.
- My medicines came from different doctors. Is that a problem?
- It can be, because neither may see the full list. That is precisely what a single pharmacy and your own written list are for — bring it to each of them.
Sources
- Primary Healthcare Standard Treatment Guidelines and Essential Medicines ListNational Department of Health, South Africa
- Medicines regulation, safety communications and adverse event reportingSAHPRA
Last reviewed 4 August 2026.