I've noticed a reduction in penile size while taking finasteride; is this a side effect and should I discontinue the medication?

Penile Size Changes on Finasteride

Penile shrinkage is not a documented side effect of finasteride in any major clinical guideline or FDA-approved labeling, and you should not discontinue the medication based on this concern alone without first exploring alternative explanations with your physician.

Documented Sexual Side Effects of Finasteride

The established sexual adverse effects of finasteride are well-characterized and do not include changes in penile size:

• Decreased libido occurs in approximately 6.4% of patients during the first year of treatment (compared to baseline rates), and this typically decreases to 2.6% in subsequent years 1, 2
• Ejaculatory dysfunction affects 3.7% in the first year, declining to 1.5% in years 2-4, and primarily manifests as decreased ejaculate volume rather than structural changes 1, 2
• Erectile dysfunction was reported in 67.4% of finasteride users versus 61.5% on placebo in the Prostate Cancer Prevention Trial, representing a relative risk of only 1.10 2
• Gynecomastia occurs in 4.5% of users versus 2.8% on placebo 2

These sexual side effects are generally reversible and become uncommon after the first year of therapy 1, 2

Why Penile Size Perception May Change

Several physiological mechanisms may create the perception of size reduction without actual anatomical shrinkage:

• Erectile quality changes: Since finasteride can affect erectile function in a subset of patients, reduced rigidity or incomplete tumescence may make the penis appear smaller during erection, though flaccid and fully erect dimensions remain unchanged 3, 4
• Prostate volume reduction: Finasteride reduces prostate size by 15-25% within 6 months, which affects internal pelvic anatomy but not penile tissue itself 5, 6
• Hormonal context: Finasteride reduces DHT by approximately 70% while increasing serum testosterone by 10-20%, but testosterone remains within normal physiologic range and is the primary hormone for erectile function—not DHT 2, 4

Post-Finasteride Syndrome Controversy

A controversial entity called "post-finasteride syndrome" has been described in case series:

• The FDA amended finasteride labels to include warnings about persistent sexual dysfunction that may continue after discontinuation, including erectile dysfunction, decreased libido, and ejaculation disorders 5
• One study found that 96% of men with persistent sexual side effects (≥3 months after stopping) continued to experience dysfunction at 14-month follow-up 3
• However, the robustness of data supporting post-finasteride syndrome remains unclear, as it is based primarily on anecdotal patient-reported outcomes rather than prospective controlled trials 5
• Importantly, even in these case series, penile size changes are not among the documented persistent symptoms 3

Clinical Approach and Common Pitfalls

Before attributing any symptom to finasteride, consider:

• Nocebo effect: Sexual adverse effects must be viewed in relation to the normal prevalence and natural history of erectile dysfunction in the population, patient age, and other confounding factors 4
• Baseline sexual function: The American Urological Association notes that each year of aging contributes 1.26 points on sexual function scales, while finasteride contributes 3.21 points 2
• Alternative causes: If you perceive worsening symptoms, consider fluid retention, weight gain affecting body proportions, or psychological factors rather than structural penile changes 2

Recommendation on Discontinuation

Do not discontinue finasteride based solely on perceived penile size changes without:

1. Documenting objective measurements with your physician to determine if actual anatomical change has occurred versus perceptual change
2. Evaluating erectile quality separately from size perception, as reduced rigidity may be the actual issue 3, 4
3. Weighing the benefits of finasteride (67% reduction in acute urinary retention and BPH-related surgery if used for BPH, or hair preservation if used for alopecia) against documented side effects 5
4. Allowing adequate time (at least 6 months) to assess whether initial side effects resolve, as most sexual adverse effects decrease substantially after the first year 1, 2

If you are experiencing bothersome erectile dysfunction or other documented sexual side effects, discuss with your physician whether the benefits of continuing finasteride outweigh these concerns in your specific clinical context 5.