Differences between a drug allergy and side effect
How to tell if you have a drug allergy
A drug allergy is your immune system’s reaction to a medicine. It’s important to be aware of allergies, because if you take the same medicine again, the reaction could be worse.
Antibiotics – particularly penicillin – are a good example. The first allergic reaction many people have is an itchy rash. But once your body is primed to recognise something as an enemy, your immune system reacts more strongly.
In worst case scenarios, it could result in a potentially life-threatening reaction called anaphylaxis. This is a sudden itchy rash – like nettle rash – that’s accompanied by wheezing, palpitations, dizziness, swelling around the mouth, the lips and the tongue, and more. Fortunately it is rare.
Rashes soon after starting a drug, are usually due to allergy, and if you develop a rash shortly after starting a medicine it should always be checked out.
If you think or you have been told you’re allergic to any medicine, tell your doctor or pharmacist before they issue it again (or an alternative drug from that group).
How to tell if it is a side effect
Side effects, rather than allergies, are far more common. Among the most common examples are:
- Feeling sick and/or diarrhoea – with antibiotics like erythromycin, statins, some painkillers and antidepressants medicines.
- Needing to pee more often – with ‘water’ tablets for high blood pressure or heart failure, and some medicines for type 2 diabetes. It is also a sign they are working.
- Tiredness – with strong painkillers, some antihistamines or depression tablets and beta-blockers for abnormal heart rhythms
- Muscle aches – with statins.
With most medicines, side effects tend to be worse just after you start the tablets, and often settle within days or weeks at most. There are exceptions. With the blood pressure-lowering ACE inhibitors, dry cough – or even swelling around the face and lips (angio-oedema) – can develop more than one year after starting.
