Human drug interaction studies in vivo are conducted when in vitro and/or animal interactions suggest clinical relevance. Studies in vitro have indicated that the new, entirely synthetic 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor fluvastatin affects the metabolism of the nonsteroidal anti-inflammatory drug diclofenac and the oral hypoglycemic tolbutamide. Diclofenac and tolbutamide are both model substrates of the CYP2C isozymes, suggesting that this enzyme could be involved in the underlying mechanism of interaction. The concomitant use of lipid-lowering drugs with oral hypoglycemic agents has been recommended in patients with non-insulin-dependent diabetes mellitus (NIDDM). Therefore, 2 studies were initiated to explore potential pharmacokinetic and pharmacodynamic interactions between fluvastatin, simvastatin, or placebo and the oral hypoglycemic agents tolbutamide (study I) and glyburide (study II), each in 16 healthy subjects. These compounds were selected because of a demonstrated in vitro interaction with tolbutamide and widespread clinical use of glyburide. A further study (study III) was conducted to investigate the potential pharmacokinetic and pharmacodynamic interactions between fluvastatin and glyburide under therapeutic conditions in 32 patients with NIDDM. Single and multiple coadministration of fluvastatin 40 mg or simvastatin 20 mg increased the mean maximum plasma concentration and area under the concentration-time curve of glyburide by about 20%. The pharmacokinetics of tolbutamide were influenced to only a minor extent. Fluvastatin concentration-time profiles were unaffected by tolbutamide or glyburide coadministration. However, the pharmacokinetic interactions between fluvastatin or simvastatin and tolbutamide and glyburide were not associated with clinically relevant changes in blood glucose and insulin concentrations and, therefore, are not considered to be relevant in therapeutic practice.(ABSTRACT TRUNCATED AT 250 WORDS)