Drug allergy: diagnosis and management

Drug allergy

All drugs can cause side effects (or 'adverse drug reactions'). These are listed in the patient information leaflet that comes with each drug. Less commonly, someone can have a drug allergy – an allergic reaction caused by the person's immune system. The NICE guideline on drug allergy only covers this type of reaction.

You can be allergic to any drug, including over‑the‑counter and prescribed drugs. You can have an allergic reaction to a drug even if you have taken it before without a problem. Some reactions happen within minutes of taking the drug, and they can appear in different ways – for example, a red itchy rash or swelling of the lips and around the eyes. Vary rarely someone can have a serious reaction called anaphylaxis. This affects breathing and blood circulation and can be life‑threatening. A drug allergy can also trigger an asthma attack in people who have asthma. If you have a severe reaction, such as anaphylaxis or an asthma attack, you need to see a doctor straight away.

Other allergic reactions can take days or weeks to appear. They may develop more quickly though if you have had a reaction to the same drug before. Often reactions affect the skin. Sometimes they may cause symptoms from problems with the blood and internal organs such as the liver and kidneys.