The great number of reactions of the skin to the administration of drugs warrants a separate discussion.

The skin is the organ most commonly affected by severe, undesirable effects of drugs, and this can occur by a variety of different mechanisms: rashes and the like, which follow the taking of drugs, are often classed as allergic, but it must be said that the evidence for allergy is frequently scanty.

Allergic reactions

In order to be able, with absolute certainty, to classify such a drug reaction as “allergic” it is essential to demonstrate that antibodies or immune-competent cells directed against an antigen have been formed from the drug or one of its metabolites, and since drugs usually consist of fairly small molecules, the allergen that causes this reaction will as a rule be a compound formed by the drug and tissue protein.

In practice, the diagnosis of an allergic skin reaction to a drug is made only in those who have previously received the drug in question on one occasion and have tolerated it, but who, on renewed treatment, develop skin lesions with or without general symptoms such as high fever. Such people may develop an allergic reaction without having received exactly the same drug previously: for example, taking one sulphonamide drug can readily cause allergy to other sulphonamides in those sensitive to them.

Anaphvlactic shock and nettle rash

These are caused by the formation of specific antibodies that become attached to tissue cells called mast cells. When an individual is again exposed to the drug, the allergen reacts with the specific antibodies, and histamines and other active substances are released from the mast cells and cause urticaria and, in some cases, a fall in blood pressure, spasms of the muscles of the bronchi (airways), and edema (swelling) of the larynx.

In severe allergy, a person can die within minutes from allergic - or anaphylactic - shock. In moderate forms of allergy, urticaria alone may occur.

Penicillin, aspirin, heparin (a drug that reduces blood clotting) and X-ray contrast media are fairly common examples of drugs that can cause these reactions.

Cwo toxic allergy

This mechanism takes place when antibodies develop against drug-protein coupling with certain cells, and the immune reaction results in cell damage. Such allergic blood disorders (thrombocytopenias, leukopenias and hemolytic anemias) are well known in general medicine.

Toxic-complex svndrome

This syndrome, also called Arthus reaction, is the result of the formation of antibodies that, on reaction with a drugprotein coupling, produce damage to blood vessels via complement, a constituent of blood serum.

When antisera were in common use in the treatment of certain infections, they often produced these reactions.
At present, penicillin is the commoThe sufferers develop pain in the joints, fever and blood disorders, and their skin becomes tender and urticaria often occurs. Delayed allergy Allergic contact eczema due to drugs applied directly to the skin is a common reaction. The same type of allergic reaction may be caused by taking the same drugs by mouth, injected into a vein, as a pessary, etc. Sulphonamides, chlorothiazide and numerous other drugs may produce this effect.