Cinacalcet hydrochloride (cinacalcet) is a positive allosteric modulator of the calcium-sensing receptor indicated for the treatment of secondary hyperparathyroidism in dialysis patients. In vitro study has demonstrated that cinacalcet is a potent inhibitor of cytochrome P450 (CYP) 2D6 with a K(i) value of 0.087 micromol/L, which is comparable to the well-known potent CYP2D6 inhibitor, quinidine (0.064 micromol/L). A clinical study was conducted to assess the inhibitory effect of cinacalcet on CYP2D6 substrates in healthy volunteers. Each subject received 50 mg of cinacalcet or a matched placebo orally once daily for 8 days with 30 mg of dextromethorphan coadministered on day 8. The mean AUC(0-infinity) and C(max) of dextromethorphan increased 11- and 7-fold, respectively, in extensive metabolizers when coadministered with cinacalcet versus placebo. Therefore, during concomitant treatment with cinacalcet, it may be necessary to consider making dose adjustments for drugs with a narrow therapeutic index that are mainly metabolized by CYP2D6.