What is Finasteride and How Does It Work?
Finasteride is a medication classified as a 5-alpha reductase inhibitor [1.5.2]. Its primary mechanism of action is to block the enzyme 5-alpha reductase, specifically the type II isoenzyme [1.5.5]. This enzyme is responsible for converting testosterone into a more potent androgen called dihydrotestosterone (DHT) [1.5.2]. By inhibiting this process, finasteride can reduce circulating serum DHT levels by approximately 70% and prostatic DHT levels by over 90% [1.5.1].
DHT is a key hormone implicated in two common conditions in men:
- Androgenetic Alopecia (Male Pattern Baldness): High levels of DHT at the scalp can cause hair follicles to shrink, leading to hair loss [1.5.5].
- Benign Prostatic Hyperplasia (BPH): DHT stimulates the growth of the prostate gland [1.3.1].
Finasteride is FDA-approved to treat both of these conditions [1.8.2]. For hair loss (Propecia®), the typical dose is 1 mg daily, while for BPH (Proscar®), a 5 mg daily dose is used [1.2.1, 1.8.2].
The "Size" Question: Prostate, Penile, and Breast Tissue
The query "Does finasteride increase size?" requires looking at three different parts of the body, with three very different answers.
Prostate Size: A Decrease
For its approved indication in treating BPH, finasteride's goal is to decrease the size of an enlarged prostate [1.3.4]. By significantly lowering DHT levels within the prostate, the medication effectively shrinks the gland's volume [1.3.3, 1.3.6]. Studies have shown that finasteride can reduce prostate volume by at least 20% within the first year of treatment and that this effect is most beneficial for men with larger prostates (greater than 30 ml) [1.3.6, 1.3.2]. This size reduction helps alleviate urinary symptoms associated with BPH, such as weak stream and frequent urination, and can reduce the need for future prostate surgery [1.3.1]. The effect lasts only as long as the medication is taken; if discontinued, the prostate will begin to grow again [1.3.1].
Penile Size: No Evidence of Increase, Anecdotal Reports of Change
There is no clinical evidence to suggest that finasteride causes an increase in penis size [1.2.1]. The medication's mechanism of action on DHT does not support a growth effect on this tissue. However, some users have reported persistent sexual side effects, including a perception of penile numbness or even shrinkage [1.7.2]. A study on rat models did show that finasteride could reduce the weight of the corpus cavernosum and decrease the cross-sectional penile area [1.2.3, 1.2.2]. It's important to note that these are findings from animal studies and that direct physical shrinkage is not a recognized common side effect in human clinical trials [1.2.1].
Breast Size: Risk of Gynecomastia
A known, though uncommon, side effect of finasteride is gynecomastia—the enlargement of male breast tissue [1.9.1]. By altering the balance between androgens (like testosterone and DHT) and estrogen, finasteride can lead to breast tenderness and enlargement [1.4.5, 1.9.2]. The risk is dose-dependent and more common with the 5 mg dose used for BPH than the 1 mg dose for hair loss [1.4.6]. In one study, the incidence of gynecomastia was 2.2% in the finasteride group compared to 0.7% in the placebo group [1.4.4]. While rare, cases have been reported even with the low 1 mg dose, and this side effect may be underreported [1.4.1, 1.4.3]. If breast tissue changes, lumps, pain, or nipple discharge occur, it is critical to consult a doctor immediately [1.9.2].
Comparison Table: Finasteride vs. Dutasteride
Dutasteride is another 5-alpha reductase inhibitor, often compared to finasteride. It is more potent, inhibiting both type 1 and type 2 isoenzymes of 5-alpha reductase [1.6.1].
| Feature | Finasteride | Dutasteride |
|---|---|---|
| Mechanism | Inhibits Type 2 5-alpha reductase [1.5.5] | Inhibits Type 1 & Type 2 5-alpha reductase [1.6.1] |
| Serum DHT Reduction | ~70% [1.6.1] | >90% [1.6.1] |
| FDA-Approved Uses | BPH, Male Pattern Hair Loss [1.8.2] | BPH [1.6.1] |
| Half-Life | 6-8 hours [1.6.1] | ~5 weeks [1.6.3] |
| Side Effect Profile | Includes sexual dysfunction, gynecomastia [1.9.5] | Similar to finasteride, though some studies note a higher incidence of decreased libido at higher doses [1.6.1] |
Other Potential Side Effects and Considerations
Beyond changes in tissue size, finasteride is associated with other potential side effects. The most commonly reported are sexual in nature, including:
- Decreased libido (sex drive) [1.9.2]
- Erectile dysfunction [1.9.2]
- Ejaculation disorders, including decreased semen volume [1.9.2]
These side effects occur in a small percentage of users (typically 2-4%) and often resolve after discontinuing the drug, though not always [1.9.5].
Post-Finasteride Syndrome (PFS) A controversial and serious consideration is Post-Finasteride Syndrome (PFS). This is a term used to describe a constellation of persistent sexual, neurological, physical, and psychological side effects that continue even after stopping the medication [1.7.1, 1.7.4]. Symptoms can include severe erectile dysfunction, depression, cognitive impairment ('brain fog'), anxiety, and penile shrinkage [1.7.1, 1.7.2]. The medical community is still working to understand the prevalence and mechanisms of PFS, but patients should be aware of this potential risk [1.7.4].
Conclusion
So, does finasteride increase size? The answer depends entirely on what tissue is in question.
- It decreases prostate size, which is its therapeutic purpose for BPH [1.3.5].
- It has no evidence of increasing penile size, with some reports of the opposite sensation [1.2.1].
- It can increase breast size (gynecomastia) as a rare but significant side effect [1.4.1].
Finasteride is an effective medication for its approved uses, but its powerful hormonal effects come with a range of potential side effects. Anyone considering this medication should have a thorough discussion with their healthcare provider about the potential benefits and risks.
Authoritative Outbound Link: For more information on finasteride from a regulatory agency, visit the U.S. Food and Drug Administration's page on Propecia [1.8.1].
