Adverse effects

Unwanted effects of drugs are commonly referred to as toxic effects or side-effects. However, these are ambiguous terms.

• Toxic effects occur through exaggeration of the desired pharmacological action of the drug, and therefore are dose related. Antipsychotic drugs produce some toxic effects by antagonism at dopamine receptors in the extrapyramidal tracts.

• Toxic effects can also occur through exaggeration of actions other than those that are thought to produce the therapeutic action; these effects are also dose related. Tricyclic antidepressants cause dry mouth, glaucoma, and urinary retention by anticholinergic action.

• Side-effects also occur through actions unrelated to the desired pharmacological effect, and so the anticholinergic actions of psychotropic drugs are toxic side-effects. An allergic reaction, which was not dose related, would be a true side-effect.

• The terms 'toxic effects' and 'side-effects' do not cover the unwanted effect of loss of drug action (e.g. through the induction of drug metabolism in a drug interaction).

Therefore it is better to use the terms 'unwanted effects' or 'adverse effects' and to classify them as shown in Tabje.3.

Table 3 Classification of adverse drug effects

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