A relationship exists between perhexiline blood concentration and toxicity, and a therapeutic range of 0.15 to 0.6 mg/L (0.5 to 2 micromol/L) has been established. Patients who remain symptomatic on perhexiline within this range may require careful dose titration to achieve plasma concentrations in the range of 0.6 to 1.2 mg/L. Monitor within 3 to 5 days of the initial starting dose to identify slow metabolisers, then every 1 to 2 weeks until stabilised (the half-life is 7 to 14 days), and thereafter every 3 to 6 months. Perhexiline is metabolised by CYP2D6, so more careful monitoring is required if drugs that affect CYP2D6 are added or stopped, as this can affect plasma concentration and the risk of toxicity, or decrease efficacy.