Transgender hormone therapy
Finasteride is sometimes used in hormone replacement therapy for transgender women due to its antiandrogenic effects, in combination with a form of estrogen. However, little clinical research of finasteride use for this purpose has been conducted and evidence of safety or efficacy is limited. Moreover, caution has been recommended when prescribing finasteride to transgender women, as finasteride may be associated with side effects such as depression, anxiety, and suicidal ideation, symptoms that are particularly prevalent in the transgender population and in others at high risk already. Adverse effects
A 2010 Cochrane review of finasteride for BPH found that, in men with a weighted mean age of 62.4, adverse effects are "rare; nevertheless, men taking finasteride are at increased risk for impotence, erectile dysfunction, decreased libido, and ejaculation disorder, versus placebo." As of 2016 fresh evidence suggested such effects, along with disturbed neurosteroid production, may persist after finasteride use is stopped.Finasteride is contraindicated in pregnancy. The Food and Drug Administration advises that donation of blood or plasma be deferred for at least one month after taking the last dose of finasteride.The FDA has added a warning to 5α-reductase inhibitors concerning an increased risk of high-grade prostate cancer, as the treatment of BPH lowers PSA (prostate-specific antigen), which could mask the development of prostate cancer.
It has common features with other self-diagnosed "mystery syndromes" such as morgellons syndrome or multiple chemical sensitivity. There is no known underlying biological mechanism for the proposed syndrome, and its incidence is also unclear. A lack of clear diagnostic criteria and the variable reporting fraction in different health-care settings make the problem challenging to evaluate.Self-reported symptoms of post-finasteride syndrome include penile atrophy and tissue changes, decreased ejaculate volume and quality, libido loss, erectile dysfunction, loss of penile sensitivity, decreased orgasm sensation, dry skin, metabolic changes, muscle and strength loss, gynecomastia, depression, anxiety, panic attacks, insomnia, anhedonia, concentration problems, memory impairment and suicidal ideation. A meta-analysis found significant association between finasteride use and post-discontinuation depression, suicide ideation, and sexual dysfunction, but the quality of evidence was limited.Plaintiffs have filed over one thousand court cases against Merck over the effects of finasteride. Most were settled by 2018 when Merck paid a lump sum of US$4.3 million to be distributed. As of September 2019, 25 cases remained outstanding in the United States.In 2019 Reuters reported that faulty redactions in court documents revealed allegations from plaintiffs that Merck had known of persistent side effects in their original clinical trials but chose not to disclose them in warning labels. Overdose
Finasteride has been studied in humans at single doses of up to 400 mg and at continuous dosages of up to 80 mg/day for three months, without adverse effects observed. There is no specific recommended antidote for finasteride overdose.