Some data indicate that bupropion inhibits the cytochrome P-450 enzyme CYP2D6, but very little published data is available on the extent of this inhibition. The objective of the present study was to quantify this inhibition in a subject treated with bupropion for smoking cessation. Genotypically, the patient was a CYP2D6 homozygous extensive metabolizer (EM). His CYP2D6 phenotype was assessed using the test drug dextromethorphan before, during, and after treatment with bupropion. During treatment with bupropion, he clearly changed from the EM to the poor metabolizer (PM) phenotype. Although the results from a single patient should be interpreted with great caution, the extent of the interaction indicates that bupropion might be a CYP2D6 inhibitor as potent as the most powerful CYP2D6 inhibitors known, such as quinidine and paroxetine.