Avodart and Rivaroxaban Interaction: What Patients and Clinicians Need to Know

Is there a known interaction between Avodart and rivaroxaban?

The Avodart drug interactions section covers potent CYP3A4 inhibitors such as ritonavir, the alpha blockers tamsulosin and terazosin, the calcium channel blockers verapamil and diltiazem, cholestyramine, digoxin and warfarin [1]. Rivaroxaban is not among them [1].

Dutasteride is a 5-alpha reductase inhibitor approved for symptomatic benign prostatic hyperplasia at 0.5 mg once daily [1]. Rivaroxaban is a direct factor Xa inhibitor, an oral anticoagulant rather than a vitamin K antagonist like warfarin.

What the Avodart label says

Dutasteride is extensively metabolized by CYP3A4 and CYP3A5. The label says the effect of potent CYP3A4 inhibitors on dutasteride has not been studied, and advises caution with potent, chronic inhibitors such as ritonavir [1]. Verapamil and diltiazem decrease dutasteride clearance and raise exposure, a change the label does not consider clinically significant; it recommends no dose adjustment [1].

The label also reports that dutasteride 0.5 mg/day for 3 weeks did not change the steady-state pharmacokinetics of digoxin, did not alter the pharmacokinetics of the S- or R-warfarin isomers or warfarin's effect on prothrombin time, and did not affect steady-state tamsulosin or terazosin levels [1]. Warfarin is not a factor Xa inhibitor, so those results do not show what happens with rivaroxaban.

The rivaroxaban side

Rivaroxaban is eliminated with a terminal half-life of 5 to 9 hours in healthy young adults and 11 to 13 hours in older adults [2]. A pharmacokinetic review reports that it does not inhibit cytochrome P450 enzymes or known drug transporter systems, and that because it has multiple elimination pathways it has no clinically relevant interactions with most commonly prescribed medications [2]. Those statements describe rivaroxaban's behavior on its own.

What is not known

Being absent from the Avodart interaction list does not establish that the combination is safe. The label gives no data on the two drugs together, so the effect of Avodart on rivaroxaban blood levels or bleeding risk is not known.