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A substrate is a substance that is acted upon and changed by an enzyme. An enzyme inducer accelerates metabolism of co-prescribed drugs which are substrates of the same enzyme, reducing their effect. An enzyme inhibitor retards metabolism of co-prescribed drugs, increasing their effects (see Chapter 7, Metabolism). Competition between drugs that are substrates for the same enzyme may retard their metabolism, increase plasma concentration and lead to enhanced therapeutic or adverse effects.

*CYP 2D6 is involved only in the breakdown of venlafaxine to its active metabolite and implications of 2D6 interactions are of limited significance.

metabolised to norfluoxetine, tl/2 200 h). The metabolic products of certain TCAs are antidepressants in their own right, e.g. nortriptyline (from ami-triptyline), desipramine (from lofepramine) and imipramine (from clomipramine).

Half-lives of TCAs lie generally in the range of 15 h (imipramine) to 100 h (protriptyline) and those for SSRIs from 15 h (fluvoxamine) to 72 h (fluoxetine).

Around 7% of the Caucasian population have very limited CYP 2D6 enzyme activity. Such 'poor metabolisers' may find standard doses of tricyclic antidepressants intolerable and it is often worth starting at a very low dose. If the drug is then tolerated, plasma concentration assay may to confirm the suspicion that the patient is a poor metaboliser.

TABLE 19.2B Psychotropic (and selected other) drugs known to be CYP 3A4 substrates. Inhibitors and inducers

CYP 3A4 inhibitors

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