Does finasteride (5-alpha-reductase inhibitor) cause increased urinary output when used for hair loss?

Finasteride and Urinary Output in Hair Loss Treatment

Finasteride used for hair loss does not cause increased urinary output; rather, it may help reduce urinary symptoms in men with enlarged prostates by decreasing prostate size.

Mechanism of Action and Urinary Effects

• Finasteride is a 5-alpha-reductase inhibitor that blocks the conversion of testosterone to dihydrotestosterone (DHT), which is responsible for both male pattern hair loss and prostate enlargement 1
• At the 1mg daily dose used for hair loss treatment, finasteride has similar effects on DHT levels as the 5mg dose used for BPH (approximately 50% reduction) 2
• Rather than increasing urinary output, finasteride has been shown to:
  • Reduce the risk of acute urinary retention by about one-third 2
  • Decrease the need for surgical interventions related to prostate issues 2

Effects on Urinary Symptoms

• Finasteride can increase peak urinary flow rate and reduce BPH symptoms, with patients experiencing an average 3-point improvement in the AUA Symptom Index 2
• The medication reduces prostate size by approximately 20%, which can alleviate urinary obstruction symptoms 2, 1
• Studies show finasteride decreases the risk of acute urinary retention from 5.6% to 3.3% (absolute risk difference of 2.3%) 2
• However, finasteride is ineffective for urinary symptoms in patients who do not have enlarged prostates 2

Considerations for Hair Loss Treatment

• The 1mg dose used for hair loss has similar effects on DHT levels as the 5mg dose used for BPH treatment, but information beyond 1 year is limited 2
• Studies specifically examining urinary effects of the 1mg dose for hair loss are limited, but one study showed no significant effects on ejaculate volume compared to placebo 3
• Small but significant decreases in prostate volume (-2.6%) and serum PSA (-0.2 ng/ml) were observed with 1mg finasteride, which reversed upon discontinuation 3

Known Side Effects of Finasteride

• The most common adverse effects are sexual in nature, not urinary:
  • Decreased libido (3% of patients initially, decreasing to 0.3% by 19-24 months) 4
  • Erectile dysfunction (4.7% of patients during first 6 months) 4, 5
  • Decreased ejaculate volume (1.4% initially, decreasing to 0.1% by 19-24 months) 4, 6
• Some patients report persistent sexual side effects even after discontinuation 5, 7
• Non-sexual side effects can include depression and gynecomastia 4, 7

Clinical Implications

• For men taking finasteride for hair loss who also have BPH symptoms, the medication may actually provide dual benefits by improving both conditions 2
• The PSA-lowering effect of finasteride (approximately 50% reduction) must be considered when screening for prostate cancer 2, 4
• The overall discontinuation rate for finasteride is approximately 15%, similar to placebo, with discontinuation due to adverse events at about 6-7% 2, 4

In conclusion, there is no evidence that finasteride increases urinary output in men taking it for hair loss. On the contrary, in men with enlarged prostates, it may actually improve urinary symptoms by reducing prostate size and decreasing the risk of acute urinary retention.