When an unusual or unexpected event, for which there is no evident natural explanation, occurs in a patient already taking a drug, the possibility that the event is drug-caused must always be considered, and may be categorised as follows:
• The patient may be predisposed by age, genetic constitution, tendency to allergy, disease, personality, habits.
• The drug. Anticancer agents are by their nature cytotoxic. Some drugs, e.g. digoxin, have steep dose-response curves and small increments of dose are more likely to induce augmented (type
A) reactions. Other drugs, e.g. antimicrobials, have a tendency to cause allergy and may lead to bizarre (type B) reactions. Ingredients of a formulation, e.g. colouring, flavouring, sodium content, rather than the active drug may also cause adverse reactions. • The prescriber. Adverse reactions may occur because a drug is used for an inappropriately long time (type C), at a critical phase in pregnancy (type D), is abruptly discontinued (type E) or given with other drugs (interactions).
Aspects of the two sections above, Classification and Causes, appear throughout the book. Selected topics are discussed below.
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