Allergic reactions to drugs are fairly rare, but almost all drugs can cause a reaction of sorts in a small number of people. Drug reactions ranging from a mild allergic reaction to a severe anaphylactic shock are possible.

Drugs are routinely tested and scrutinised before they appear on the market. If any significant number of the people on whom the drugs are tested, have an allergic reaction, the drug will not be released.

Some people are intolerant to drugs and may develop any of the following symptoms:

 
• Itching;  
• Rashes or hives;  
• Headaches;  
• Nausea.

These are seen as mild reactions and can usually be treated by antihistamines, or by merely reducing or discontinuing the medication.

Reactions can be either immediate or take a while to develop. Previous exposure to the drug could mean increased severity of reaction every time the drug is used.

A far more severe reaction is called anaphylactic shock, which could be fatal. This happens very rarely, but if one develops any of the following symptoms, immediate medical attention is called for:

 
• All-over itchiness;  
• Swelling of the face and ears;  
• Constriction of the airway;  
• Increased heart rate;  
• Fainting.

The most common drug allergy is a reaction to drugs in the penicillin family. Other drugs that can trigger allergic reactions are sulphas, barbiturates, insulin, local anaesthetics and dyes injected into blood for X-ray purposes.

If a patient develops an allergy to medication, his GP must be alerted, he must wear a Medic Alert bracelet at all times and must get his GP to check routinely the contents of all prescription medication or over-the-counter medication.