What is a drug allergy?

There is more than one type of drug allergy. Here we focus on the allergic reactions that come on very quickly and cause hives (also known as nettle rash or urticaria), swelling (angioedema) or anaphylaxis. These reactions can start within minutes of taking the drug, or sometimes after a few hours.

This type of allergy happens when a person’s immune system reacts inappropriately to a particular drug, creating antibodies known as Immunoglobulin E (IgE). Doctors refer to this kind of allergy as “IgE- mediated”.

Many people experience delayed allergic reactions that don’t involve IgE antibodies. Symptoms usually begin more than 24 hours after taking the medication, but can start as early as two to six hours afterwards.

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Who might have a drug allergy?

Anyone can have an allergic reaction to a drug, not just 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.

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 94% of children thought to have had an allergic reaction to a drug could in fact take the drug without having a reaction (Rebelo Gomes et al, 2008).

If you have symptoms that appear to have been caused by a drug, tell your GP or pharmacist. Drug allergies are complex so it’s usually important to be referred to a specialist so they can investigate the cause of your symptoms.

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.

What are the symptoms of a drug allergy?

Mild symptoms can include: hives (also known as nettle rash or urticaria) anywhere on the body; a tingling or itchy feeling in the mouth; lip or face swelling, which is generally not serious by itself but can be a sign of something more serious if you have any of the more severe symptoms listed here at the same time.

In extreme cases, there could be a dramatic fall in blood pressure (anaphylactic shock). The person may become weak and floppy and may have a sense of something terrible happening. This may lead to collapse and unconsciousness.

If you have asthma, the risk of having severe symptoms is higher.

More severe symptoms are known as anaphylaxis. Symptoms of anaphylaxis may include:
  • right_arrow_orange_icon Swelling of the throat, tongue or mouth
  • right_arrow_orange_icon Difficulty breathing
  • right_arrow_orange_icon Severe wheezing
  • right_arrow_orange_icon Severe abdominal pain, nausea and vomiting

What to do if you think you have a drug allergy

If you think you may be allergic to a drug, avoid that drug until you have spoken to your GP, even if your symptoms have been mild.

You may need to be referred to a specialist in managing drug allergies to find out what’s going on and make a diagnosis. This may be at an allergy clinic in a hospital. Your GP can find an allergy clinic in your area through the British Society for Allergy and Clinical Immunology website.

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.

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.

Severe symptoms include:

  • breathing difficulties (whether these are caused by asthma or swelling in the airways)
  • feeling faint or weak, which may be caused by a drop in blood pressure.


These symptoms can progress fast so it’s best to play it safe and call 999 if you are at all concerned.

Treatments for drug allergies

Once you have been diagnosed with a drug allergy, it’s usually straightforward to avoid that drug. This includes telling health professionals about your allergy so they know what to avoid.

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 necessary 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 an auto-injector. Your GP or allergy specialist will advise you on this.

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 increase the amount until your immune system can tolerate the drug.

How drugs are taken and how this affects allergic reactions

By injection

The most serious reactions tend to follow injections, especially those given straight into the veins (intravenous injections). This is because the drug is quickly carried around the body 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, airway obstruction 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, which translates to one in every 10,000 procedures (Mirakian et al,2009).

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 taken 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 begin many hours after being taken.


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 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. Discuss this with your doctor or allergy specialist, plus read more about egg allergies and vaccines.

The MMR vaccination has been demonstrated to be safe for children with egg allergy, even though it’s normally cultured on cells from chick embryos (BSACI, 2007, and British National Formulary, 2013). 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.” (Clark et al 2010).”


If there is concern that someone may react to a vaccine, it can be given in hospital.



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 continued 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. Antibiotics are among the drugs most likely to cause allergy, but it is also common for people to be wrongly diagnosed with an allergy because the symptoms, such as a rash, are sometimes caused by the infection being treated rather than the antibiotic.

If the rash is immediate or widespread, or if there are 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. It’s 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 is thought to have caused the reaction. 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. If they can’t advise you, they will be able to refer you to a specialist.

Remember, if symptoms are severe or appear to be progressing rapidly, call 999.


Pain killers (analgesics)

This 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 painkillers and anti-inflammatory drugs, report these to 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

Drugs used in general anaesthesia pose a particularly difficult problem because you will generally be asleep when the reaction starts. This means it’s up to the anaesthetist to watch for symptoms such as falling blood pressure and airway obstruction. 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 have any reason to believe you could be allergic to anaesthesia, discuss this with your anaesthetist at the pre-assessment stage before surgery. You should also mention any food allergies 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 general anaesthesia. These include induction agents to make you unconscious, neuromuscular blockers which temporarily have a paralysing effect,  antibiotics, painkillers and blood or plasma substitute infusions. Any one of these could cause anaphylactic reactions so, 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 then it’s important to have an early referral to a hospital department with experience of anaesthetic-related reactions. This should be the responsibility of the anaesthetist. You may have a skin prick test, intradermal testing and sometimes a drug challenge as part of the diagnosis.

The most common cause of allergic reactions during general anaesthesia is the neuromuscular blockers. In some cases, the anaesthetic drugs may not be responsible for the symptoms at all, for example if the person is having a reaction to the latex used in the gloves or medical equipments, or to antibiotics given alongside the anaesthetic.


Local anaesthetics

Local anaesthetics, for example during dental surgery, are a rare cause of anaphylactic reactions. Sudden loss of consciousness, a drop in blood pressure, or heart irregularity may suggest an anaphylactic reaction. However, skin testing followed by a challenge test is 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 know exactly which drug was responsible and, if possible, which alternatives are safe.
  • Information about your drug allergies needs to be prominently recorded in your primary care and hospital notes.
  • Even more importantly, we advise that you make yourself responsible for telling any medical professionals who treat you about your allergy, such as doctors, nurses, dentists and pharmacists.
  • We strongly advise you to wear a medical alert bracelet or pendant, or carry a letter from a doctor explaining your allergy.
  • Serious reactions to a drug should be treated as a medical emergency. If the symptoms are severe or appear to be progressing quickly, dial 999.


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