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

    Vaccines are used to prevent and treat infectious diseases and for desensitising people who are allergic to allergens including insect venom, pollen and cats.

    If you are allergic to egg , be aware that certain vaccines such as the seasonal flu vaccine and yellow fever vaccine can contain small amounts of egg protein. You can discuss this with your doctor or allergy specialist, plus read more about egg allergies and vaccines.

    The MMR vaccine has been shown to be safe for children with egg allergy even though it’s normally cultured on cells from chick embryos. A 2010 medical paper written by UK experts said:

    “All children with egg allergy should receive their normal childhood immunizations, including the MMR vaccination, as a routine procedure performed by their family doctor/nurse… Studies on large numbers of egg-allergic children show there is no increased risk of severe allergic reactions to the vaccines. Children who have had documented anaphylaxis to the vaccine itself should be assessed by an allergist.”

    If your doctor is concerned that you or your child may react to a vaccine, it can be given in hospital.

  • Insulin

    Insulin, which is used for diabetes, has the potential to trigger allergic reactions, but anaphylaxis is rare. This may be because the injections are usually used for life so people develop a degree of tolerance. Insulin-induced anaphylaxis may be more common in people whose treatment is interrupted or intermittent.

  • Antibiotics such as penicillin

    Antibiotics are used to treat infections. They are among the drugs most likely to cause allergy. It is also common for people to be wrongly diagnosed with an allergy to antibiotics because the symptoms of an allergy can be similar to symptoms caused by the infection, such as a rash.

    If the rash comes on immediately after taking the antibiotic or is widespread, or you have other symptoms such as facial swelling or breathing problems, these are likely to be caused by an allergy. If this happens, your doctor can refer you to an allergy clinic for a skin prick test and intradermal testing, which are generally more helpful than blood tests.

    If you have been told you are allergic to penicillin, it is useful to try to find out why, and the reason may be included in your medical records. It’s also helpful to know which penicillin was involved. People with penicillin allergy can become non-allergic after many years without exposure to it, but this must always be confirmed by a specialist.

    Occasionally, people who are allergic to one antibiotic may react to another within the same ‘family’ of antibiotics. Talk to your GP or pharmacist about whether you need to avoid antibiotics in the same family. They will refer you to a specialist if needed.

  • Pain killers (analgesics) and anti-inflammatories

    There is a large group of drugs with pain-killing and anti-inflammatory properties, known as the ‘aspirin-like drugs’ or as ‘non-steroidal anti-inflammatory drugs’ (NSAIDs). They include aspirin and ibuprofen. If you have any symptoms caused by these drugs, tell your GP.

    If you have had a reaction to aspirin or ibuprofen you should be considered sensitive to the other drugs in this group (such as diclofenac) until it is proven otherwise.

    Paracetamol is not an NSAID and evidence suggests that most people who are sensitive to aspirin are able to take paracetamol. If you become allergic to aspirin or ibuprofen, and you’re not sure whether you may also react to paracetamol because you haven’t taken it for some time, you could ask your GP about being referred to a specialist. They can do a paracetamol challenge under medical supervision to test whether you are sensitive to it. If you do react to paracetamol, your doctor should help you find an alternative painkiller.

  • General anaesthetics

    If you have an allergic reaction to a general anaesthetic, you will generally be asleep when the reaction starts. This means the anaesthetist will watch for symptoms such as falling blood pressure and difficulty breathing. Fortunately, sensitivity to these drugs is rare, but because they are injected straight into the bloodstream, symptoms may be severe and progress quickly.

    If you believe you could be allergic to anaesthesia, tell your anaesthetist at the pre-assessment stage before surgery. Mention any food allergies you have in case there are any food derivatives in the drugs being used.

    It’s common for several drugs to be given together or in quick succession when you have a general anaesthetic. These include:
    • induction agents to make you unconscious
    • neuromuscular blockers which have a temporary paralysing effect – these are the most common cause of reactions
    • antibiotics
    • painkillers
    • blood or plasma substitute infusions.

    Any of these could cause anaphylaxis. If you have a reaction, it is important to find out which one caused it and which alternatives are likely to be safe in future.

    If you have a reaction that’s thought to be caused by the general anaesthetic, your anaesthetist should refer you to a hospital department with experience of anaesthetic-related reactions. You may have a skin prick test, intradermal testing and sometimes a drug challenge as part of the diagnosis.

    Sometimes, the anaesthetic drugs may not be responsible for the allergic reaction, for example, it might be due to the latex used in the gloves or medical equipment.

  • Local anaesthetics

    Local anaesthetics, for example during dental surgery, are a rare cause of anaphylactic reactions.

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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