A medication side effect is an unwanted effect that happens at a normal dose, alongside the effect you wanted. Almost every medicine has them, because medicines act on processes that occur in more than one part of the body. A medicine that reduces stomach acid affects digestion elsewhere. One that calms the nervous system also causes drowsiness. The unwanted effect is usually the same mechanism showing up somewhere you did not want it.
Three things are often confused with each other. A side effect happens at a normal dose. An allergic reaction is the immune system responding to the medicine, which can be severe and usually means avoiding that medicine in future. An overdose is harm from too much. See side effect or allergic reaction and overdose symptoms.
Common side effects and what they may feel like
Common examples linked to what are medication side effects
A small group recurs across very different medicines, because it reflects how the body handles anything new: nausea and digestive upset, headache, drowsiness or difficulty sleeping, dizziness when standing, dry mouth, and tiredness. Many settle within days to a few weeks as the body adjusts. See common medication side effects and how long they last.
Side effects are also not automatically bad news. Some are neutral, such as a harmless change in urine colour. Occasionally one becomes useful in its own right, which is how several medicines came to be used for a second purpose.
How to describe frequency without overclaiming
Leaflets group effects as very common, common, uncommon, rare and very rare. Those bands come from how often something was recorded in trials. They describe a population, not you, and they say nothing about severity: a rare effect can be an emergency and a very common one can be trivial.
The list on a leaflet is also not a prediction. It records what was observed while people took the medicine, which includes coincidences. See how to read the side effects section of a leaflet.
Serious side effects and red flags
Red-flag symptoms
Get emergency help immediately for:
- Difficulty breathing, wheeze, or a tightening throat
- Swelling of the face, lips, tongue or throat
- Chest pain, fainting or collapse
- Seizure, or sudden confusion or unresponsiveness
- A rash that blisters or peels, or sores in the mouth or eyes with fever
- Vomiting blood, or black tarry stools
- Thoughts of harming yourself
When to contact a doctor, pharmacist, or emergency service
Same day for any of these:
- Yellowing of the eyes or skin, dark urine, or pale stools
- Passing much less urine than usual, or marked swelling of the legs
- Fever with a new rash, or fever with mouth ulcers
- Unexplained bruising, or bleeding that is new for you
- A fast, slow or irregular heartbeat that is new
- Severe or worsening abdominal pain
- New muscle pain with weakness, or unusually dark urine
Risk factors, interactions and who should be cautious
People who may need extra caution
- Older adults. Slower clearance and more medicines at once. See side effects in older adults.
- Kidney or liver conditions. Reduced clearance means an ordinary dose can behave like a larger one.
- Pregnancy and breastfeeding. Ask before starting or stopping anything, including supplements.
- Children. Dosing is by weight and some medicines are unsuitable. See children and over-the-counter medicines.
- People taking several products. Risk rises with the number taken, including anything bought without a prescription.
- Anyone with a previous drug reaction. A past reaction can indicate risk with related medicines.
Medicine and supplement interactions to check
Why one person gets an effect and another does not comes down to dose, age, kidney and liver function, other medicines, genetics, and the condition being treated. Combinations matter more than most people expect:
- Supplements with prescription medicines, including St John’s Wort
- Supplements with blood thinners
- Over-the-counter medicines alongside chronic medication
- Two products that both contain paracetamol
- Alcohol with chronic medication
What to do if you think this is a side effect
Common effects
Note when the medicine started and when the symptom began, keep taking it unless told otherwise, and ask a pharmacist whether it is expected. See how to know if a medicine is causing your symptoms.
Serious effects
Act on the red flags above rather than researching further, and take the packaging with you.
When to speak to a doctor or pharmacist
Red-flag symptoms
The emergency list applies whatever the medicine and however long you have taken it.
When to contact a doctor, pharmacist, or emergency service
A pharmacist is the quickest first contact in South Africa for anything that is not urgent. See when to see a doctor.
Related side-effect guides
Common effects
- Medication side effects hub
- Serious medication side effects
- What to do if a medicine causes side effects
- Side effect symptoms index
- Report a side effect in South Africa
Serious effects
See what an adverse drug reaction is for the term used in reporting.
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.