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What's the difference between a side effect and a drug allergy?

What's the difference between a side effect and a drug allergy?

Nearly half of UK adults take at least one regular medicine - last year in the UK, over a billion prescriptions were issued! That's an awful lot of medicines, and an awful lot of potential for side effects as well as allergies.

Many of my patients get confused between side effects or intolerance and allergy. Side effects may settle with time and there may be steps you can take to reduce the risk. But if you have an allergic reaction, you must stop the medicine and never take it again.

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Are you allergic?

It's important to be aware of allergies because if you take the same medication again, the reaction could be much worse.

Antibiotics - particularly penicillin - are a good example. The first allergic reaction many people experience 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 scenario, it could result in a potentially life-threatening reaction called anaphylaxis - sudden itchy rash (like nettle rash), wheezing, palpitations, dizziness, swelling around the mouth, the lips and the tongue and more.

Rashes are usually due to allergy and if you develop a rash shortly after starting a medicine it should always be checked out.

If you've ever been told you're allergic to any medication, tell your doctor or pharmacist before they issue a new medication.

Could it be a side effect?

Side effects, rather than allergies, are far more common. Among the most common ones in my surgery (from a very long list!) are:

  • Feeling sick and/or diarrhoea (eg with antibiotics like erythromycin, statins, some painkillers and antidepressants medicines).

  • Needing to pee more often (eg with 'water' tablets for high blood pressure or heart failure, and some medicines for type 2 diabetes).

  • Tiredness (eg with strong painkillers, some antihistamines or depression tablets and beta-blockers for abnormal heart rhythms).

  • Muscle aches (eg with statins).

With most medications, side effects tend to be worst 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 (called angio-oedema) can develop over a year after starting.

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Read the leaflet (but don't panic!)

It's worth talking with your pharmacist and reading the patient leaflet for a full list of recognised side effects. However, it's important not to panic when you read the full list. Don't forget that many side effects are very rare - they all have to be listed for legal reasons, even if 99.9% of people don't get them.

Do check the label of your medicine - some tablets may cause side effects if taken on an empty stomach but be fine with food.

Ask the pharmacist

I'm a huge fan of community pharmacists - and this is yet another time they can be invaluable. Your community pharmacist really is the expert where medication is concerned - they have a university degree in it!

They can tell you if a symptom is likely due to your medicine, whether it's likely to settle and what you should do. They can also advise on possible interactions between drugs you're taking. Do remember that herbal remedies and even food supplements can cause side effects or interact with prescribed medicines - do let your pharmacist know everything you're taking.

If you're experiencing any symptoms you think might be related to your medication, your pharmacist should be your first port of call unless it's a medical emergency. As with a GP appointment, it helps to be prepared. Make a list of any symptoms you have as well as any medications you take - and that includes medication bought over the counter. Even anti-inflammatory drugs like ibuprofen, available without prescription, can cause tummy pain and even bleeding.

Depending on what medicines you take, you may be able to book a review of your medicines with your pharmacist using Patient Access.

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What about 'natural' remedies?

All too many people assume that 'natural’ supplements' or remedies are safe and don't have side effects - but everything that has a positive effect can have a negative effect as well. The list is endless, but common culprits include St John's wort, used for depression, and aspirin (whether prescribed or taken for pain).

If you're taking the blood-thinner warfarin, commonly used if you have an abnormal heart rhythm called atrial fibrillation, you need to be particularly careful. It interacts with a host of prescribed medications, supplements and even food and drink (including alcohol, Brussels sprouts and curly kale!).

It's important to have regular blood tests when you're taking warfarin, but it's also a good idea to have a chat with your pharmacist about possible interactions, especially if your blood tests aren't stable.

With thanks to My Weekly magazine, where this article was originally published.

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The information on this page is peer reviewed by qualified clinicians.

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