Finasteride pharmacokinetics

Pharmacokinetics Finasteride is well absorbed after oral administration, with a mean bioavailability of approximately 64%, which is unaffected by food. 

Steiner JF. Clinical pharmacokinetics and pharmacodynamics of finasteride. Clin Pharmacokinet 1996 30(l) 16-27. 

Because finasteride and dutasteride are metabolized primarily by CYP3A4, the CYP3A4 inhibitors, such as ritonavir, ketoconazole, verapamil, diltiazem, cimetidine, and ciprofloxacin, may increase the drugs blood levels and, possibly, cause drug-drug interactions. Clinical drug interaction studies have shown no pharmacokinetic or pharmacodynamic interactions between dutasteride and tamsulosin or terazosin, warfarin, digoxin, and cholestyramine. 

Samara E, Hosmane B, Locke C, Eason C, Cavanaugh J, Graraieman GR Assessment of the pharmacokinetic-pharmacocfynamic interaction between terazosin and finasteride. J Clin Pharmacol (199Q 36,1169-78. 

Vashi V, Chung M, Hilbert J, Lawrence V, Phillips K. Pharmacokinetic interaction between finasteride and terazosin, but not finasteride and doxazosin. J Clin Pharmacol (1998) 38, 1072-6. 

Finasteride 5 mg daily for 10 days caused no change in the pharmacokinetics or pharmacodynamics of a single 80-mg dose of propranolol in healthy subjects. Further, the manufacturers say that finasteride was used with beta blockers in clinical studies without any evidence of an interaction. Similarly, dutasteride , (p.l2S7) does not appear to interact with beta blockers. 

Phenobarbital and phenytoin reduce the serum levels of tirilazad mesilate whereas ketoconazole increases them. Finasteride inhibits the metabolism of tirilazad to its active metabolite. No pharmacokinetic interaction appears to occur between cimetidine or nimodipine and tirilazad. 

Finasteride does not appear to affect the pharmacokinetics of digoxin. 

In a randomised study, 17 healthy subjects were given a single 400-microgram dose of digoxin while taking finasteride 5 mg daily for 10 days. Finasteride had no significant effect on the pharmacokinetics of digoxin. No adverse effects are expected on concurrent use. 

Lundahl A, Hedeland M, Bondesson U, Knutson L, Lennernas H. The effect of St John s wort on the pharmacokinetics, metabolism and biliary excretion of finasteride and its metabolites in healthy men. Eur J Pharm Sci 2009 36 433-43. 

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