Finasteride and Simvastatin Interaction: What You Need to Know

What the finasteride label says

The Propecia (finasteride 1 mg) prescribing information covers this question in two short parts. The first says that no drug interactions of clinical importance have been identified and that finasteride does not appear to affect the cytochrome P450-linked drug-metabolizing enzyme system. Antipyrine, digoxin, propranolol, theophylline and warfarin were tested in people, and no clinically meaningful interactions were found [1]. The second part lists medicines that were taken at the same time in clinical studies. It says specific interaction studies were not performed, but finasteride was used with acetaminophen, aspirin, alpha-blockers, ACE inhibitors, anticonvulsants, benzodiazepines, beta blockers, calcium channel blockers, diuretics, H2 antagonists, HMG-CoA reductase inhibitors, NSAIDs and quinolone antibiotics without evidence of clinically significant adverse interactions [1].

HMG-CoA reductase inhibitors are statins, and simvastatin is one of them. The Proscar (finasteride 5 mg) label carries the same interaction wording [4]. So the closest thing to a direct statement about this pair is that statins were used alongside finasteride in clinical studies without evidence of clinically significant adverse interactions.

Why CYP3A4 keeps coming up

Simvastatin is metabolized through CYP3A4 [2]. Finasteride is also metabolized mainly by CYP3A4, although the label's interaction section does not discuss CYP3A4 inhibitors or inducers [1]. Being cleared by an enzyme, however, is not the same as blocking it. The finasteride label states that finasteride does not appear to affect the cytochrome P450 system [1]. Two drugs can both pass through the same enzyme without either one changing the other's levels.

What a documented simvastatin interaction looks like

The classic CYP3A4 example for simvastatin is grapefruit juice. In a randomized crossover study of 10 healthy volunteers, 200 mL of double-strength grapefruit juice three times daily for two days raised the area under the curve of unchanged simvastatin 16-fold and of simvastatin acid about 7-fold after a 60 mg dose [2].

Statin muscle injury is also more likely at higher doses and when certain other medicines are taken at the same time [3]. Finasteride appears in the label only among the medicines taken concurrently in trials without clinically significant adverse interactions [1].

Muscle symptoms and what they mean

Myopathy means muscle damage, and it is the simvastatin hazard that matters when its blood levels rise. In rare cases myopathy occurs with statin therapy, especially at higher doses and with certain other medicines [3].

What is not known

No dedicated pharmacokinetic study of finasteride with simvastatin is cited here, so the effect on simvastatin exposure has not been quantified. The reassurance comes from the finasteride label's general interaction statement and from statins having been used in finasteride clinical studies [1].