Sparteine oxidation is practically abolished in quinidine‐treated patients

In eight patients a sparteine‐test was carried out immediately before and after 1 week of treatment with quinidine 600‐800 mg day‐1. Before treatment one patient was classified as a poor metaboliser (metabolic ratio: greater than or equal to 20), and seven patients as extensive metabolisers. During quinidine treatment, the formation of sparteine metabolites (2‐ and 5‐dehydrosparteine) was practically abolished. Patients initially classified as extensive metabolisers thus exhibited the phenotype of poor metabolisers during quinidine treatment.