05.01.2.2 Pharmacokinetic Interactions

The enzyme CYP3A4 metabolises 50% of all drugs, and this is source of many drug interactions. The anti-fungal drug ketoconazole inhibits CYP3A4, and this increases plasma levels of drugs metabolised by CYP3A4, of which there are many. For instance, sildenafil (Viagra) is metabolised by CYP3A4. The combination of ketoconazole and sildenafil leads to increased levels of sildenafil. Lower doses of sildenafil are used in men taking ketoconazole.