Why do medications cause allergic reactions? | Children's Charity

Why do medications cause allergic reactions?

This research was completed on 31 March 2009

Published on 31 January 2007

Allergy to drugs is a common problem. It can cause extremely severe and even fatal reactions. It can also mean people have to stop taking a drug they really need, such as an antibiotic or a treatment for epilepsy. Researchers are investigating what causes these allergies, in the ultimate hope of predicting who is at risk and identifying how we might help them.

What's the problem and who does it affect?

Allergies to drugs can be life-threatening

Estimates suggest allergic reactions to drugs are responsible for up to 1 in every 240 admissions to hospital in the UK.1,2 Allergic reactions can cause severe symptoms, which are sometimes life-threatening. Despite the dangers of drug allergies, we can’t predict who is at risk of developing them. The skin is the most commonly affected tissue. Most allergies to drugs cause relatively minor skin rashes and hives. But people with severe allergies can suffer blisters and extensive loss of skin similar to that caused by a bad burn. They often have to be treated in intensive care. The liver, lungs and kidneys may also be damaged, and some people suffer an extremely severe reaction called anaphylaxis, which requires emergency treatment. Medications that can commonly cause allergic reactions include painkillers, antibiotics and anti-epileptics. Once a person has become allergic to a particular drug, they cannot receive it again without significant risk. They have to try alternatives, which may not work as well. But people who’ve suffered a severe allergic reaction often become very anxious about taking any kind of medication.

What is the project trying to achieve?

What makes the immune system see drugs as harmful?

People develop an allergy when their immune system sees the drug as a dangerous invader within the body, and launches an attack against it. But no-one knows exactly how this happens – most people’s immune systems see medicines as harmless. The researchers working on this project aim to define differences in the immune responses of people who become allergic and people who do not. Preliminary data suggests that allergies may result from inappropriate behaviour by a particular group of immune cells, called regulatory T cells. Researchers hypothesise that regulatory T cells normally stop the immune system from reacting to a drug, and that allergies result from the failure of these cells to prevent an immune response. Researchers are testing their hypothesis through sophisticated studies of the behaviour of immune cells taken from blood samples. The samples are being donated by allergic, and non-allergic, people who have been taking the antibiotic amoxicillin, or the anticonvulsants phenytoin or carbamazepine, as well as people who have not been taking these drugs.

What are the researchers' credentials?

Project LeaderProfessor P S Friedmann MD, FRCP, FMedSci
Project team
  • Dr Christopher Pickard PhD
  • Prof Eugene Healy FRCP
LocationDermatopharmacology Unit, University of Southampton
DurationTwo years
Grant awarded31 October 2006
Start date1 February 2007
End date31 March 2009
Grant amount£88,393.00
Grant codeSP4202

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Professor Peter Friedmann, who is leading the project team in Southampton, is a world expert in allergic processes that involve the skin and a leading expert in the field of drug allergy. Southampton School of Medicine is a world-renowned centre for allergy research and the dermatology team have made important contributions towards proving how the immune system becomes involved in allergy to medicines. The team is equipped with state-of-the-art facilities for analysis of immune mechanisms. They have already identified several groups of patients with drug allergies who have agreed to take part in past and future studies – a resource that makes them particularly well placed to succeed with this research.

Who stands to benefit from this research and how?

Cellular mechanisms underlying allergies

Researchers hope to reveal detailed new information about how the immune system reacts to medications. They hope to explain why some people become allergic to a particular drug while others do not, by revealing differences in the behaviour of immune cells. Researchers believe that increasing our understanding of the underlying cellular mechanisms that make some people susceptible to allergies may, in the future, lead to the development of tests that can predict who is at risk of having an allergic reaction to particular drugs. People who are at risk would then be able to avoid taking medications that might harm them. The project team estimates that as many as 15,000 patients could benefit each year in the UK in due course.1,2 The NHS could be saved significant amounts of money, because admissions to hospital for allergic reactions may, in theory, be greatly reduced. Treating just one patient with a severe allergy that requires intensive care costs the researchers’ hospital around £15,000. Looking even further into the future, researchers believe continuing their line of investigation may ultimately lead to new treatments that could control allergic reactions to medicines, so that patients who’ve developed an allergy to a drug they really need can carry on taking it.


1. Pirmohamed M, James S, Meakin S, Green C, Scott AK, Walley TJ, Farrar K, Park BK, Breckenridge AM. Adverse drug reactions as a cause of admission to hospital: prospective analysis of 18,820 patients. BMJ 2004;329:15-19. 2. Chan HL, Stern RS, Arndt KA et al. The incidence of erythema multiforme, Stevens-Johnson syndrome, and toxic epidermal necrolysis. A population-based study with particular reference to reactions caused by drugs among outpatients. Arch Dermatol 1990;126:43-7.

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