Adverse reactions to drugs may result from several causes. These may be related to the usual pharmacological properties of the drug, to drug over-dose, to intolerance to the drug or due to a true allergy against the drug.

A drug allergy is an abnormal immunological reaction to a drug mechanism.  Reactions typically occur at low concentrations of the drug and occur rapidly after administration of the drug. Typical features of a drug allergy include a widespread, itchy, red, urticarial rash; tight chest, and swelling of the throat and face. An allergic reaction to a drug may progress on to Anaphylaxis, a shock-like state in which the blood pressure is also lowered, and the patient loses consciousness

Immune reactions to drugs may occur immediately or may be delayed occurring a day or two after exposure. These are mediated by cellular mechanisms, or other non-IgE antibodies. These usually present with more persistent maculo-papular (flat) rashes, fever, joint pain, and swollen glands. Non IgE maculo-papular rashes typically occur following ampicillin administration.

Some drugs cause an allergic eczematous contact dermatitis. This typically occurs as an occupational hazard in individuals who manufacture or dispense drugs. Some drugs may also cause contact dermatitis when used therapeutically on the skin

Other drugs cause skin reactions when the skin is exposed to ultraviolet light during the time when the patient is taking the drug. These reactions may be activated by “sunburn” wave lengths (285 – 310 nm) or by the longer wave lengths (320 – 450 nm) which cause photo allergic reactions

Adverse reactions to medications may also result from a preservative, stabilizer or colouring agent associated with the drug, such as in the drug tablet (Table 4). Aspirin and other salicylates typically cause adverse reactions such as bronchospasm, angioedema and urticaria in sensitive individuals. Such individuals are also sensitive to other non-steroidal anti-inflammatory drugs (Table 5).

Investigation of Drug Allergy

The diagnosis of a drug allergy is usually made from the history of typical clinical features occurring after exposure to the drug.

a)Skin Prick Tests
Skin tests are exceptionally reliable for the confirmation of allergy to penicillin, toxoids, insulin, egg protein vaccines, latex and muscle relaxants.
Skin Prick Tests should only be carefully performed by specialists in emergency room settings.

B) Laboratory tests
The ImmunoCAP RAST Test may be performed to confirm the presence of IgE antibodies to Penicillin, Insulin, Alcuronium, Suxamenthonium, Amoxycillin, Thiopentone, Protamine and some of the Cephalosporins and several other drugs. CAP RAST tests for drugs may not be positive in all sensitive individuals and a specialist interpretation is usually necessary.

The CAST Test is a new test that measures the sensitivity of the patient’s cells to a drug or preservative or colourant, in the laboratory. This test may also detect non-IgE sensitivity.

What to do if you have a drug allergy?

1. Inform your medical doctor and your family.
2. Wear a Medic-Alert bracelet.
3. Take great care that you do not inadvertently receive the drug.
4. Be careful to check the name of the active ingredient when you receive a generic tablet.
5. Study the labels on over the counter medicines that you may take.
6. Avoid all the other drugs that are in the same chemical grouping as the drug to which you are allergic. You should obtain a detailed list of these from your doctor, or from the local DRUG INFORMATION CENTRE or your pharmacist.
7. Always inform your anaesthetist should you require surgery.