Getting to know

Drug allergy can be expressed in a variety of forms, skin eruptions being the most common and anaphylaxis the most severe. 

Anaphylaxis is the most feared form of drug allergy because of its possible fatal consequences. Fortunately, anaphylactic reactions are not very common. The most dangerous consequences of drug anaphylactic reactions are swelling of the throat or larynx (which block the air passages and can lead to cutting off of the oxygen supply) and the sudden enlargement of the blood vessels (which in very severe cases may cause a catastrophic fall in blood pressure that can result in death). 

Other symptoms that may occur are urticaria (hives), itching of the skin, flushing, or even asthma symptoms (more frequent and more severe in people who already suffer from asthma). In most cases anaphylaxis symptoms start abruptly within the first hour (very often within the first 30 minutes) after taking the drug and can become severe very quickly. Generally, it seems that the longer the delay in onset, the milder the anaphylactic attack is. The most common cause of drug induced anaphylaxis is antibiotics (particularly penicillin and penicillin related drugs). 

Aspirin and other anti-inflammatory drugs and dyes used for radiological investigation can also cause anaphylaxis like symptoms. However in these cases there is a pseudoallergic reaction rather than a true allergy.

Skin eruptions are the most common expression of drug allergy. 
Most commonly drug reactions show as a skin rash, very similar to the ones in children’s contagious diseases (i.e. measles, rubella, scarlet fever etc). Very often (but not always) the eruption is itchy. Usually these rashes are mild and disappear within a few days after the treatment is stopped. However, if the skin starts peeling or if blisters develop consult your physician immediately. These are signs of a severe and possibly dangerous outcome. 

Urticaria (itchy wheals, like nettle stings, that change location quickly) and angioedema (swelling of the lips, tongue, mouth, throat, larynx) are also frequent consequences of drug allergy. These symptoms can be caused by a true drug allergic reaction (the main cause being penicillin and penicillin like antibiotics) and also by a pseudoallergic reaction (manly caused by aspirin and related anti-inflammatory drugs). Urticaria and angioedema can occur as the only symptoms or they can be part of an anaphylactic reaction. 

Allergic contact dermatitis  (eczema) can develop with drugs applied on the skin (gels, lotions, creams, ointments). It manifests initially with rash which then evolves into blisters that very often break open giving rise to skin erosions and crusts. Usually there is also intense itching. 

Most frequently allergic contact eczema is related to the use of antibiotic products applied on the skin (neomycin is probably the most common cause as it is the antibiotic most frequently used topically on the skin) and also to anaesthetics contained in topical skin products for the treatment of sun burns, itch or pain. Allergy to drug products applied on the skin can be caused not only by the drug itself but also by other substances, like preservatives or fragrances, contained in the product. 

Sometimes drug induced eczema can appear only if the skin is exposed to sun or ultraviolet radiation (like the one in the skin tanning beauty salons).