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Is Finasteride a Blood Thinner? No, Here's Its Drug Profile

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At a glance

  • Drug class / Type II 5-alpha reductase inhibitor, not an anticoagulant or antiplatelet [1]
  • Label interaction statement / No drug interactions of clinical importance identified [1][2]
  • Warfarin / Tested in people; no clinically meaningful interaction found [1]
  • Other tested drugs / Antipyrine, digoxin, propranolol, theophylline and warfarin [1]
  • Alpha-blocker combination / Proscar plus doxazosin is an approved indication to reduce symptomatic BPH progression [2]
  • PSA effect / Proscar lowers PSA about 50% within six months; an isolated PSA after six months should be doubled [2]
  • Metabolism / Main pathway is CYP3A4 [1]
  • Forms / 1 mg Propecia for male pattern hair loss; 5 mg Proscar for BPH [3][4]

Is finasteride a blood thinner?

No. Finasteride is a competitive inhibitor of type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone [1]. It is prescribed as Propecia 1 mg for male pattern hair loss and as Proscar 5 mg for symptomatic BPH [1][2][3][4]. It is not an anticoagulant or antiplatelet medicine.

The Propecia and Proscar labels state that no drug interactions of clinical importance have been identified [1][2]. Finasteride does not appear to affect the cytochrome P450-linked drug-metabolizing enzyme system [1].

What drug interactions does the finasteride label list?

The labels name five compounds tested in people: antipyrine, digoxin, propranolol, theophylline and warfarin. No clinically meaningful interactions were found [1].

The Propecia label's other-concomitant-therapy section lists acetaminophen, acetylsalicylic acid, alpha-blockers, analgesics, angiotensin-converting enzyme inhibitors, anticonvulsants, benzodiazepines, beta blockers, calcium channel blockers, cardiac nitrates, diuretics, H2 antagonists, HMG-CoA reductase inhibitors, NSAIDs and quinolone anti-infectives as medicines used with finasteride in clinical studies without evidence of clinically significant adverse interactions [1].

Does finasteride interact with warfarin or aspirin?

Warfarin is one of the compounds the Propecia label lists as tested in people, with no clinically meaningful interaction found [1]. The Proscar label reports the same statement [2].

The Propecia label also lists acetylsalicylic acid (aspirin) and NSAIDs among medicines used with finasteride in clinical studies without evidence of clinically significant adverse interactions [1].

Does finasteride interact with alpha-blockers for BPH?

The Proscar label includes Proscar with the alpha-blocker doxazosin as an indicated combination to reduce the risk of symptomatic progression of BPH, defined as a confirmed at least 4-point increase in AUA symptom score [2].

The Medical Therapy of Prostatic Symptoms (MTOPS) trial enrolled 3,047 men and compared placebo, doxazosin, finasteride and combination therapy [5]. Combination therapy reduced the risk of overall clinical progression by 66% versus placebo; finasteride alone reduced it by 34% and doxazosin alone by 39% [5]. This evidence uses Proscar 5 mg with doxazosin, not Propecia 1 mg [2][5].

The Proscar label's MTOPS safety table reports postural hypotension in 17.8% of the combination group versus 8.0% with placebo and 16.7% with doxazosin alone, and dizziness in 23.2% versus 8.1% with placebo and 17.7% with doxazosin alone [2].

How does finasteride affect PSA?

Finasteride changes PSA, a blood test, not clotting. The Proscar label says PSA falls by approximately 50% within six months, and an isolated PSA after six months should be doubled for comparison with normal ranges in untreated men [2]. Any confirmed increase from the lowest PSA value while on Proscar may signal prostate cancer and should be evaluated, even if the value is still within the normal range for men not taking a 5-alpha reductase inhibitor [2].

For Propecia 1 mg, the label reports mean PSA fell from 0.7 to 0.5 ng/mL at month 12 in men aged 18 to 41, and any confirmed increase from the lowest value while on Propecia should be evaluated, even if still within the normal range [1].

What about CYP3A4 inhibitors and inducers?

Finasteride is metabolized mainly by CYP3A4 [1]. The drug interaction sections of the Propecia and Proscar labels do not describe studies with CYP3A4 inhibitors such as ketoconazole or with inducers, and they do not give a finasteride dose adjustment for those medicines [1][2]. The compounds named as tested in people are antipyrine, digoxin, propranolol, theophylline and warfarin [1].

What about minoxidil, testosterone and statins?

A 12-month randomized trial in 450 Chinese men with male pattern hair loss assigned 160 men to finasteride 1 mg, 130 to 5% topical minoxidil and 160 to both [6]. At 12 months, improvement was 80.5% with finasteride, 59% with minoxidil and 94.1% with the combination; adverse reactions were reported in 1.8% with finasteride and 6.1% with minoxidil [6].

Finasteride blocks the conversion of testosterone to DHT [1]. The Propecia label reports mean circulating testosterone and estradiol rose by about 15% and stayed within the normal range [1].

The Propecia label lists HMG-CoA reductase inhibitors among medicines used with finasteride in clinical studies without evidence of clinically significant adverse interactions [1].

Frequently asked questions

Is finasteride a blood thinner?
No. Finasteride is a type II 5-alpha reductase inhibitor, not an anticoagulant or antiplatelet drug [1]. Its labels state that no drug interactions of clinical importance have been identified [1][2].
Does finasteride interact with warfarin or aspirin?
Warfarin is one of the compounds tested in people taking finasteride, and no clinically meaningful interaction was found [1]. The Propecia label also lists aspirin and NSAIDs as used in clinical studies without evidence of clinically significant adverse interactions [1].
Does finasteride interact with alpha-blockers like doxazosin?
The Proscar label includes Proscar with doxazosin as an indicated combination to reduce symptomatic BPH progression [2]. In MTOPS, combination therapy reduced overall clinical progression by 66% versus placebo, compared with 34% for finasteride alone and 39% for doxazosin alone [5]. The label's MTOPS table shows more postural hypotension and dizziness with combination therapy than with placebo, finasteride alone or doxazosin alone [2].
Does finasteride affect PSA test results?
Yes. The Proscar label says PSA falls by about 50% within six months and an isolated PSA after six months should be doubled for comparison with normal ranges in untreated men [2]. For Propecia 1 mg, mean PSA fell from 0.7 to 0.5 ng/mL at month 12, and any confirmed increase from the lowest value should be evaluated [1].
Can finasteride and minoxidil be used together?
A 12-month randomized trial in 450 men compared finasteride 1 mg, 5% topical minoxidil and the combination [6]. Improvement was 80.5% with finasteride, 59% with minoxidil and 94.1% with the combination [6]. The labels state that no drug interactions of clinical importance have been identified [1][2].
What about ketoconazole and CYP3A4?
Finasteride is metabolized mainly by CYP3A4 [1]. The labels do not describe studies with CYP3A4 inhibitors such as ketoconazole or with inducers. They state that no drug interactions of clinical importance have been identified and name antipyrine, digoxin, propranolol, theophylline and warfarin as compounds tested in people [1][2].

References

  1. U.S. National Library of Medicine, DailyMed. Propecia (finasteride 1 mg) tablets, Organon prescribing information. Propecia (finasteride 1 mg) tablets, Organon label
  2. U.S. National Library of Medicine, DailyMed. Proscar (finasteride 5 mg) tablets, Organon prescribing information. Proscar (finasteride 5 mg) tablets, Organon label
  3. Drugs@FDA: FDA-Approved Drugs. Drugs@FDA: FDA-Approved Drugs
  4. Drugs@FDA: FDA-Approved Drugs. Drugs@FDA: FDA-Approved Drugs
  5. McConnell JD, Roehrborn CG, Bautista OM, Andriole GL, Dixon CM, Kusek JW, et al. The long-term effect of doxazosin, finasteride, and combination therapy on the clinical progression of benign prostatic hyperplasia. N Engl J Med. 2003;349(25):2387-98. The long-term effect of doxazosin, finasteride, and combination therapy on the clinical progression of benign prostatic hyperplasia
  6. Hu R, Xu F, Sheng Y, Qi S, Han Y, Miao Y, et al. Combined treatment with oral finasteride and topical minoxidil in male androgenetic alopecia: a randomized and comparative study in Chinese patients. Dermatol Ther. 2015;28(5):303-8. Combined treatment with oral finasteride and topical minoxidil in male androgenetic alopecia: a randomized and comparative study in Chinese patients
Evidence overview for Is Finasteride a Blood Thinner? No, Here's Its Drug Profile