An adverse drug effect is defined as 'any unwanted effect of treatment from the medical use of drugs that occurs at a usual therapeutic dose'. Almost every drug can cause an adverse reaction, which must be elicited in the history. Any substance that produces beneficial therapeutic effects may also produce unwanted, adverse or toxic effects. The severity of the reaction may range from a mild skin rash or nausea to sudden death from anaphylaxis. A study has shown that the incidence of adverse reactions increases from about 3% in patients 10-20 years of age to about 20% in patients 80-89 years of age. 1 Reactions can be classified in several ways, e.g. side effects, overdosage, intolerance, hypersensitivity and idiosyncrasy. However, a useful classification of unwanted effects is divided into type A and type B. Type A reactions are the most common and involve Augmented pharmacology, i.e. they are caused by unwanted, albeit predictable, effects of the drug. Examples
• constipation due to verapamil
• blurred vision and urinary outflow problems due to tricyclic antidepressants
• hyperuricaemia due to thiazide diuretics
Type B reactions are by definition Bizarre. The reactions are unpredictable from known properties of the drug. Examples include hepatotoxicity and blood dyscrasias.
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