When a new drug is introduced, it has usually been studied only in relatively few patients – typically 1,500. If n patients have been studied, and no serious eﬀects observed, there is still a chance of a serious adverse eﬀect occurring in the general population as frequently as 3/n (1:500).
Adverse eﬀects can be divided into types. First, those which are closely related to the concentration of the drug and accord with what is known of its PHARMACOLOGY. These so-called type A (augmented pharmacological) eﬀects are distinguished from type B (bizarre) eﬀects which are unpredictable, usually rare, and often severe. ANAPHYLAXIS is the most obvious of these; other examples include bone-marrow suppression with CO-TRIMOXAZOLE; hepatic failure (see HEPATITIS) with SODIUM VALPROATE; and PULMONARY FIBROSIS with AMIODARONE. A more comprehensive classiﬁcation includes reactions type C (chronic eﬀects), D (delayed eﬀects – such as teratogenesis or carcinogenesis) and E (end-of-dose eﬀects – withdrawal eﬀects). Examples of adverse reactions include nausea, skin eruptions, jaundice, sleepiness and headaches.
While most reported adverse reactions are minor and require no treatment, patients should remind their doctors of any drug allergy or adverse eﬀect they have suﬀered in the past. Medical warning bracelets are easily obtained. Doctors should report adverse eﬀects to the authorities – in the case of Britain, to the Committee on Safety of Medicines (CSM), using the yellow-card reporting machinery.