Finasteride Drug Interactions
Finasteride has minimal clinically significant drug interactions, with no documented interactions of clinical importance reported in major clinical trials. 1, 2, 3
Evidence from Clinical Trials
The most robust safety data comes from the Proscar Long-Term Efficacy and Safety Study (PLESS), which followed 3,040 men aged 45-78 years for 4 years. No drug interactions of clinical importance were observed in finasteride-treated patients, including in older men (≥65 years) taking concomitant medications for multiple comorbidities. 3
Pharmacokinetic Profile Supporting Low Interaction Risk
Finasteride's pharmacokinetic properties explain its favorable interaction profile:
- Finasteride undergoes extensive hepatic metabolism to essentially inactive metabolites, which are eliminated through bile and urine. 2
- The drug is widely distributed but exerts pharmacological effects specifically on tissues containing high concentrations of 5α-reductase (prostate, scalp, genital skin), limiting systemic interactions. 2
- No dosage adjustments are necessary for patients with renal dysfunction, as metabolites that accumulate are relatively inactive and well-tolerated. 2
Combination Therapy Evidence
Finasteride has been extensively studied in combination with alpha-blockers without significant drug interactions:
- The MTOPS trial combined finasteride with doxazosin in 3,047 men over 4-6 years with no reported drug interactions. 4
- The PREDICT trial combined finasteride with doxazosin (4-8 mg) in 286 European men without interaction concerns. 4
- The VA Cooperative Study combined finasteride with terazosin (10 mg) in 272 men without documented interactions. 4
- Studies combining finasteride with tamsulosin (0.2-0.4 mg) showed no interaction issues. 4
Special Considerations in Older Males with Comorbidities
For older males with BPH and multiple comorbidities, finasteride remains safe:
- Studies included men up to age 89 years receiving finasteride without age-related interaction concerns. 5
- Tamsulosin (often co-prescribed) is well tolerated in elderly patients with multiple comorbidities. 5
- Within both older (≥65 years) and younger age cohorts, no significant differences were found between placebo and finasteride-treated patients in cardiovascular adverse events. 3
Important Clinical Caveats
PSA Monitoring Interaction
Finasteride suppresses PSA levels by approximately 50% after 12 months of therapy. 6 When monitoring for prostate cancer:
- Double the PSA value in patients on finasteride to establish the appropriate baseline. 4
- Failure to adjust PSA values can lead to delayed cancer diagnosis and worse outcomes (higher Gleason scores, more advanced stage at diagnosis). 4
Ophthalmologic Consideration
If cataract surgery is planned, inform the ophthalmologist about tamsulosin use (if co-prescribed) due to increased risk of intraoperative floppy iris syndrome. 5 This is not a finasteride interaction but relevant for combination therapy.
Food Effects
While food may slow the rate of finasteride absorption, it has no effect on total bioavailability, so the drug can be taken without regard to meals. 2
Monitoring in Real-World Practice
Regular monitoring should focus on:
- Symptom scores and post-void residual volumes at regular intervals. 5
- Adjusted PSA values (doubled) for cancer surveillance. 4, 6
- Sexual function, as finasteride increases risk of erectile dysfunction, decreased libido, and ejaculation disorders compared to placebo, though these are drug effects rather than interactions. 1, 7