9%) had been prescribed either fluoxetine or paroxetine (CYP2D6-inhibiting antidepressants) and 33,897 (10.2%) had been prescribed the non-CYP2D6-inhibiting antidepressants (fluvoxamine, citalopram, venlafaxine, and sertraline) at some time during metoprolol therapy. Within the cohort of older patients receiving metoprolol, we identified 8232 cases hospitalized with bradycardia. Of these, 99 were newly treated with a study antidepressant in the 30 days preceding the

index date. Within this group, 23 (23.2%) had received either fluoxetine or paroxetine (strong Inhibitors,research,lifescience,medical inhibitors of CYP2D6) and 76 (76.8%) had received one of fluvoxamine, citalopram, venlafaxine, or sertraline (Table 1). Table 1. Characteristics of cases and controls. Table 2 outlines the univariate and multivariate analyses for the risk of bradycardia following exposure Inhibitors,research,lifescience,medical to CYP2D6-inhibiting antidepressants Enzastaurin purchase relative to noninhibiting antidepressants. The univariate OR estimate of relative risk of bradycardia was 0.78 (95% CI 0.47–1.31; p = 0.35) (Table 2). The adjusted relative risk estimate was similar (OR 0.76; 95% CI 0.42–1.37; p = 0.37). We found similar results in the sensitivity analysis excluding sertraline, with no increased risk of bradycardia evident following initiation of paroxetine or fluoxetine (OR 1.01; 95% CI 0.53–1.94; p = 0.98). Table 2. Results Inhibitors,research,lifescience,medical from logistic regression of bradycardia outcome.* Discussion

Using population-based health services research methods, we found that in a large cohort of older patients receiving metoprolol, the addition of a CYP2D6-inhibiting antidepressant did Inhibitors,research,lifescience,medical not appreciably increase the risk of hospitalization for bradycardia compared with a non-CYP2D6-inhibiting antidepressant. To our knowledge, this is the first study using population health data to assess objective clinical consequences of the potential drug–drug interaction between metoprolol and antidepressants Inhibitors,research,lifescience,medical that inhibit CYP2D6. The absence of an elevated risk

of bradycardia with fluoxetine and paroxetine is unexpected given the in vitro and human subject evidence that these antidepressants increase serum levels of metoprolol [Alfaro et al. 2000; Belpaire et al. 1998; Hemeryck et al. 2001; Walley et al. 1993; Yoon et al. 2000]. The importance of our findings is Terminal deoxynucleotidyl transferase highlighted by the high frequency of concomitant treatment with SSRI antidepressants and metoprolol in our sample. Overall, 14% of patients used any antidepressant, and 3.9% were exposed to one that inhibits CYP2D6. Concern about adverse events from drug–drug interactions is appropriate given evidence of an interaction [Alfaro et al. 2000; Belpaire et al. 1998; Hemeryck et al. 2001; Walley et al. 1993; Yoon et al. 2000] and the likelihood for exposure. The high antidepressant prescription rate is similar to other studies describing increasing use of antidepressants [Olfson and Marcus, 2009; Raymond et al. 2007].