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

Last year in England, more than 1 billion prescriptions were issued. That's an awful lot of medicines, and an awful lot of potential for side effects as well as allergies.

#MedSafetyWeek is an annual campaign organised by the Medicines and Healthcare products Agency (MHRA). This encourages you to report any suspected side effects from medicines, a medical device incident, or if you believe a product isn't working as intended. The MHRA Yellow Card Scheme helps to detect and resolve potential safety issues with medicines and healthcare products.

#MedSafetyWeek 2024 (4-10 November)

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.

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 medicine, tell your doctor or pharmacist before they issue a new medicine.

How to tell if it's a side effect?

Side effects, rather than allergies, are far more common. Among the most common examples are:

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.

Patient picks for Allergies

Allergies, blood & immune system

When to worry about an allergic reaction

Allergies are complicated. They happen when your immune system, which usually helps your body fight off infection and other intruders, over-reacts. They're also very common - affecting around one quarter of people in the UK at some point. About half of people with allergies are children but you can develop an allergy at any age. While many are mild - an itchy nose from mild hay fever is one of the commonest - others can be life-threatening. So how do you know the difference?

by Dr Sarah Jarvis MBE, FRCGP

Allergies, blood & immune system

Antihistamines

Antihistamines are a group of medicines which act to block the effects of the chemical called histamine in the body. Either H1 or H2 histamine receptors can be blocked by medicines, but the group commonly known as antihistamines blocks the H1 receptor. They have a number of uses, but are most often used to treat allergies.

by Dr Colin Tidy, MRCGP

Read the leaflet

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

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

Ask the pharmacist

I'm a huge fan of community pharmacists. This is yet another time they can be invaluable. Your community pharmacist really is the expert where medication is concerned.

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, so let your pharmacist know everything you're taking.

If you're having any symptoms you think might be related to your medicine, speak to your pharmacist first, 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 medicines you take - and that includes medicines bought over the counter. Even anti-inflammatory drugs like ibuprofen, available without prescription, can cause tummy pain and bleeding.

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

What about 'natural' remedies?

Many people assume that 'natural’ supplements or remedies are safe and don't have side effects. However, everything that has a positive effect can have a negative effect as well. The list is endless, but some examples 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 medicines, 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.