Drug Profile

In the 1990s, the Food and Drug Administration (FDA) approved two doses of finasteride — an oral prescription drug that belongs to a drug class called 5-alpha reductase inhibitors, which inhibit the formation of a potent form of testosterone — for use in men only. The high (5-milligram [mg]) dose, which is sold under the brand name PROSCAR, is approved for use as a stand-alone drug to control symptomatic benign prostatic hyperplasia or enlargement (BPH) and also for use in combination with the alpha-blocker doxazosin (CARDURA) to reduce the risk of symptomatic progression of this condition.[2]

The low-dose (1-mg) form of finasteride, which is sold under the brand name PROPECIA, is approved for male-pattern hair loss.[3]

Neither dosage form of the drug has been approved for use in women or children. Both dosage forms are taken once daily.

For many years, Public Citizen’s Health Research Group has classified both finasteride products as Do Not Use because their benefits do not outweigh their adverse effects. Yet the drug continues to be among the top 100 most commonly prescribed medications in the U.S.[4]

Small benefits

The main clinical trials that supported the initial approval of finasteride showed that the drug improved some symptoms of BPH (mainly resulting in increased urine output) compared with a placebo. A 1993 review by the independent French drug safety journal Prescrire International characterized these benefits as “slow” and as being of “mild clinical efficacy.”[5] The review also noted a lack of long-term clinical trials at that time comparing finasteride with alpha-blockers (such as doxazosin), which are the first-line drugs for BPH.

A 2010 review by Cochrane (a nonprofit organization that specializes in analyzing evidence from multiple studies) concluded that doxazosin improves BPH symptoms better than finasteride in the short and long term.[6] Notably, doxazosin (and other alpha-blockers) improve BPH symptoms immediately by relaxing the smooth muscles in the bladder and prostate, whereas finasteride generally takes six to 12 months before it improves BPH symptoms.[7]

A 2003 trial with an average follow-up of 4.5 years found that taking either doxazosin or finasteride, as well as taking both of them together (in combination), improves BPH symptoms, but only their combination decreased clinical progression of the disorder.[8] The 2010 Cochrane review also showed that combining doxazosin and finasteride improves urinary symptoms better than doxazosin alone in men with medium or large prostates but does not do so in those with small prostates.[6] However, the review found that this combination is associated with more adverse effects than is taking doxazosin alone.

Likewise, the clinical trials that supported the approval of finasteride for male pattern hair loss showed that the drug increased hair growth in one-half of the male subjects with mild-to-moderate degrees of male pattern baldness after one year of treatment.[3] However, any hair that grew due to finasteride use was shed after the drug was discontinued.

Adverse effects

Finasteride is associated with serious adverse effects that were not known until several years after the drug was approved, requiring the addition of multiple warnings to the drug’s labeling over the years.

A large, federally funded clinical trial, called the Prostate Cancer Prevention Trial, found that finasteride increases the risk of high-grade prostate cancer (although the drug reduced the risk of overall prostate cancer) compared with placebo.[9] Therefore, an editorial that accompanied a 2003 New England Journal of Medicine (NEJM) article that summarized the results from that trial argued that finasteride should not be used for prostate cancer prevention.[10] The results of this trial also led the FDA to require that a warning be added to the labeling of all finasteride products regarding this increased risk of high-grade prostate cancer among users of the drug.[11]

An 18-year follow-up of the Prostate Cancer Prevention Trial that was published in the NEJM also found an increased risk of high-grade prostate cancer but no difference in survival between finasteride users and those who received a placebo.[12] However, a 2019 analysis of large population-based data from the Department of Veterans Affairs showed that finasteride was associated with delayed prostate cancer diagnosis and worsened prostate cancer outcomes among men who took the drug at least one year before being diagnosed with prostate cancer.[13]

Evidence from the Prostate Cancer Prevention Trial and other postmarketing studies also links finasteride to sexual adverse effects including decreased libido, erectile dysfunction, infertility and reduced ejaculate volume.[9] Postmarketing studies show that these sexual adverse effects persist in some patients after finasteride is discontinued.[14],[15],[16]

An analysis that linked data from the Prostate Cancer Prevention Trial with Medicare claim data showed a 10% increase in depression-related claims among finasteride users compared with those who received a placebo.[17]

Evidence from postmarketing and population-based studies has linked finasteride to suicidal thoughts (ideation),[18],[19],[20],[16] which prompted French drug regulators to require this adverse effect to be added to finasteride labeling in early 2018.[21] In contrast, the FDA has not, as of December 2021, taken a similar action to warn U.S. consumers about this risk.

Finasteride also has been linked to additional neuropsychiatric adverse effects that are not yet included on its labeling, including anxiety, decreased concentration, memory problems and slowed thought processes.[22] These adverse effects possibly occur because the drug has negative effects on the levels of multiple hormones in the brain, including those derived from progesterone and cortisol, which influence mood and memory.

Additionally, finasteride can cause other physical adverse effects, including hearing impairment, male breast enlargement (gynecomastia), metabolic problems, muscle weakness and skin rash.[23]

Persistence of a combination of the mental, neurological, physical and sexual adverse effects among patients who have taken finasteride is known as post-finasteride syndrome.[24],[16] Although this condition is still not recognized as an official medical condition, it is an area of ongoing research.

Other adverse effects of finasteride include angioedema (a potentially life-threatening allergic reaction that can cause swelling in the skin, lips, mouth and throat), male breast cancer and chronic fatigue.[25],[2],[26] Importantly, finasteride causes abnormalities in the external genitalia of male fetuses of pregnant women who are exposed to the drug.[2]

Other research has suggested a link between finasteride use and seizures [27] as well as an increased risk of type 2 diabetes.[28]