About 15 per cent of the UK population report adverse reactions to medication, but only five per cent are truly allergic reactions. Adverse reactions usually involve the immune system. Of these less than one per cent are fatal.

• Those not related to the drug but coincidental and due to factors other than the drug. For example, rashes or nausea associated with the disease and not the medication.
• Toxic effects: from taking an overdose; those due to known side effects; and those due to interactions with other medication taken at the same time.
• Then there are allergic reactions and these adverse reactions usually involve the immune system.

What are the causes?
These may be due to direct anaphylactic immune reactions such as seen with penicillin, vaccines, blood transfusions, insulin and intravenous fluids.

Triggers include aspirin and anti-inflammatory drugs

Other medicines can trigger histamine release in the body by non-immune mechanisms and for which no diagnostic blood tests are available. Triggers include aspirin and anti-inflammatory drugs, morphine and the opiates, local anaesthetics and some fluids given intravenously during x-ray.

Drugs that cause allergic reactions:

• antibiotics - penicillin, sulphonamides, chloramphenicol and cephalosporins
• heart drugs - ACE inhibitors, quinidine, amiodarone, methyldopa
• anaesthetic drugs - muscle relaxants, thiopentone, halothane
• morphine derivatives - morphine, pethidine and codeine
• aspirin-like drugs - diclofenac, ibuprofen, indomethacin
• cancer chemotherapy drugs - cisplatin, cyclophosphamide, methotrexate
• antiseptics - chlorhexidine, iodine
• vaccines such as tetanus toxoid and diphtheria vaccine
• preservatives such as Parabens and Benzakonium chloride
• anticonvulsants, antituberculosis medication, streptokinase, insulin, enzymes and latex

What are the symptoms?
The reaction may progress to life threatening anaphylaxis.

Most reactions occur within one hour and involve measles-like itchy rashes or urticaria. A severe form may even blister (Stevens-Johnson Syndrome). The reaction may progress to life threatening anaphylaxis. Delayed reactions can develop days after exposure to the drug with generalised dermatitis and damage to organs such as the kidneys, liver, lungs and blood cells.

What is the treatment?
Treatment involves immediate withdrawal of the implicated drug, followed by antihistamine medication. In cases of anaphylaxis, the prompt use of adrenaline and steroids is life saving.

A blood test will confirm if an allergic reaction has taken place. Unfortunately only penicillin, amoxycillin, sulphonamide and cephalosporin allergy can be reliably confirmed by RAST testing. If any other drug is suspected then skin testing, followed by challenge tests in a hospital will be needed. Patch tests on the skin can test allergy to certain drugs such as neomycin and the paraben preservatives.

What can be done to prevent drug allergies?
If you are allergic to a member of family of drugs such as penicillin or aspirin, then all other members of that family should be avoided unless negative skin or challenge tests have been performed. And if you're allergic to penicillin, use the Erythromycin family of antibiotics instead. The same goes for anti-inflammatory medication such as ibuprofen - only use paracetamol.

It is possible to have an allergic reaction to almost any drug - including paracetamol - so only use medication if absolutely necessary or if it has been specifically prescribed to you. Never use someone else's medication unless you have taken medical advice and are sure it is safe to use.