What is a drug allergy?

There are different types of drug allergy. Here we focus on the type of reactions that come on very quickly and can cause hives (also known as nettle rash or urticaria), swelling (angioedema) or anaphylaxis (the most serious type of reaction). These reactions usually start within minutes of taking the drug but sometimes after a few hours.

This type of allergy happens when the body’s immune system wrongly identifies a particular drug as a threat and creates antibodies known as Immunoglobulin E (IgE). Antibodies are proteins that form part of the immune system and are made to protect you from substances that don’t belong in the body. Doctors call this kind of allergy ‘IgE- mediated’.

Many people have a different type of drug allergy where the reaction comes on later and does not involve IgE antibodies. Symptoms usually begin more than 24 hours after taking the drug, but can start as early as two to six hours afterwards. We don’t cover these types of reactions here.

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Who might have a drug allergy?

Anyone can have an allergic reaction to a drug, not only people with other allergies such as hay fever or food allergies.

Drug allergies are most common in adults, especially the elderly. Often, the person will have taken the drug before but not had a reaction.

What are the symptoms of a drug allergy?

The symptoms of a drug allergy can come on quickly, within minutes of taking the drug.

Mild to moderate symptoms may include:

• nettle rash (known as hives or urticaria) anywhere on the body
• a tingling or itchy feeling in the mouth
• swelling of lips, face or eyes
• stomach pain or vomiting.

More serious symptoms are often referred to as the ABC symptoms and can include:
  • right_arrow_orange_icon AIRWAY - swelling in the throat, tongue or upper airways (tightening of the throat, hoarse voice, difficulty swallowing).
  • right_arrow_orange_icon BREATHING - sudden onset wheezing, breathing difficulty, noisy breathing.
  • right_arrow_orange_icon CIRCULATION - dizziness, feeling faint, sudden sleepiness, tiredness, confusion, pale clammy skin, loss of consciousness.

The term for this more serious reaction is anaphylaxis (anna-fill-axis).

In extreme cases, there could be a dramatic fall in blood pressure. The person may become weak and floppy and may have a sense of something terrible happening. This may lead to collapse and unconsciousness and, on rare occasions, can be fatal.

Most healthcare professionals consider an allergic reaction to be anaphylaxis when it involves the ABC symptoms. Read more about anaphylaxis.

If you have asthma, and it is not well controlled, this could make an allergic reaction worse. Make sure you discuss this with your GP or allergy specialist and take any prescribed medicines.

Getting a diagnosis

If you think you may be allergic to a drug, tell your GP or pharmacist. If you need specialist help, your GP can find an allergy clinic in your area from the British Society for Allergy and Clinical Immunology (BSACI).

Drug allergies are complex so it’s usually important to be referred to a specialist so they can find out what’s going on and make a diagnosis.

Avoid taking the drug until you have spoken to your GP, even if your symptoms have been mild.
Once you have a diagnosis, you can take steps to minimise the risk of having another reaction in the future. You will need to learn about your allergy and, if you are at risk of a serious reaction, wear a medical alert bracelet or pendant at all times.

Whenever you see a health professional such as a doctor, dentist or pharmacist, always mention your drug allergy. It might be in your medical records but it could be overlooked.

If you have an adverse reaction to a drug, report it to the Medicines and Healthcare Products Regulatory Agency (MHRA) as soon as possible.

Other causes of symptoms can include:
  • right_arrow_orange_icon Infection: sometimes symptoms that are thought to be caused by an allergy are actually caused by an infection, not by the drug being used to treat it.
  • right_arrow_orange_icon Side-effects: side effects of drugs can cause similar symptoms to allergies, such as a skin rash.

Treatments for drug allergies

Once you have been diagnosed with a drug allergy, it’s usually straightforward to avoid that drug. Tell any health professionals you see about your allergy so they know what to avoid prescribing.

People who are allergic to other things such as food or insect stings are sometimes prescribed adrenaline auto-injectors to carry with them at all times. This isn’t usually needed for drug allergies because it’s unlikely you’ll take the drug without knowing it, although there are special cases where you might need to carry adrenaline auto-injectors. Your GP or allergy specialist will be able to advise you.

If you need a specific drug but you’re allergic to it, and there is no safe alternative available, a special technique called desensitisation can be used. This is where you take small amounts of the drug under strict medical supervision and take larger doses over time until your immune system can tolerate the drug.

What to do if you have a reaction

If you have mild symptoms such as a minor rash or flushing of the skin, tell your GP or pharmacist as soon as possible. It can be helpful to take photographs of symptoms such as swelling or skin rash to show them.

If the symptoms are becoming severe or appear to be progressing quickly, you or someone you’re with should call 999 straight away.

How drugs are taken and how this affects allergic reactions

By injection
The most serious allergic reactions tend to follow injections, especially those given straight into the veins (intravenous injections). This is because the drug is carried around the body very quickly in the blood.

If you are allergic to the drugs given as part of a general anaesthetic that have a paralysing effect, signs such as flushing, difficulty breathing or a drop in blood pressure can appear within seconds, usually within three minutes. Approximately 500 people have serious allergic reactions to anaesthetic drugs every year in the UK, that’s one in every 10,000 procedures. Your anaesthetist will be looking out for the signs of a reaction and will be ready to treat it.

Injections given beneath the skin (subcutaneous) or into a muscle (intramuscular) can cause a local reaction at the injection site, including reddening, swelling (weals) and itching.

By mouth
Reactions to drugs that you take by mouth can start within minutes but might happen up to two hours later if the drug is absorbed slowly. Some delayed-release drugs may cause reactions that come on many hours later.

Through the skin
Antibiotics or other drugs applied to burns or inflamed or damaged skin may cause allergic reactions. On rare occasions they can cause anaphylaxis.

Types of drugs that can trigger allergic reactions

These drugs are the main ones to be aware of, although there may be others.

  • Vaccines

  • Insulin

  • Antibiotics such as penicillin

  • Pain killers (analgesics) and anti-inflammatories

  • General anaesthetics

  • Local anaesthetics

Is it really an allergy

Sometimes, symptoms appear to be caused by a reaction to a drug but are in fact caused by something else. One study showed that more than nine out of ten children thought to have had an allergic reaction to a drug could in fact take the drug without having a reaction.

Other causes of symptoms can include:

  • Infection: sometimes symptoms that look like an allergic reaction are actually caused by an infection, not by the drug.
  • Side-effects: side effects of drugs can cause similar symptoms to allergies, such as a skin rash.

 

After local anaesthetics, people sometimes have a sudden loss of consciousness, a drop in blood pressure, or a change in heart rhythm which looks like anaphylaxis. But skin tests and challenge tests are often negative, suggesting the symptoms may have been caused by something else, such as a fainting reaction.

Key messages

  • If you are allergic to a drug, it’s vital to find out which drug caused the reaction and, if possible, which alternatives are safe.
  • Your drug allergies needs to be recorded in your primary care and hospital notes, in a prominent place.
  • Always tell any medical professionals who treat you about your allergy, including doctors, nurses, dentists and pharmacists.
  • Wear a medical alert bracelet or pendant if you are at risk of a serious allergy, or carry a letter from a doctor explaining your allergy.
  • Serious reactions to a drug are a medical emergency. If the symptoms are severe or appear to be progressing quickly, call 999.

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