Side effects of finasteride include impotence (1.1% to 18.5%), abnormal ejaculation (7.2%), decreased ejaculatory volume (0.9% to 2.8%), abnormal sexual function (2.5%), gynecomastia (2.2%), erectile dysfunction (1.3%), ejaculation disorder (1.2%) and testicular pain. Resolution occurred in men who discontinued therapy with finasteride due to these side effects and in most of those who continued therapy.
In December 2008, the Swedish Medical Products agency concluded a safety investigation of Propecia and subsequently advised that the use of Propecia may result in irreversible sexual dysfunction. The Agency's updated safety information lists difficulty in obtaining an erection that persists indefinitely, even after the discontinuation of Propecia, as a possible side effect of the drug.
The UK's Medical and Healthcare Products Regulatory Agency (MHRA) say that erectile dysfuction that persists once use of Propecia has stopped has been reported to them.
Finasteride is not indicated for use by women. Finasteride is in the FDA pregnancy category X. This means that it is known to cause birth defects in an unborn baby. Women who are or who may become pregnant must not handle crushed or broken finasteride tablets, because the medication could be absorbed through the skin. Finasteride is known to cause birth defects in a developing male baby. Exposure to whole tablets should be avoided whenever possible, however exposure to whole tablets is not expected to be harmful as long as the tablets are not swallowed.
It is not known whether finasteride passes into breast milk, and thus should not be taken by breastfeeding women. Finasteride may pass into the semen of men, but Merck states that a pregnant woman's contact with the semen of a man taking finasteride is not an issue for concern. Finasteride is known to affect blood donations, and potential donors are typically restricted for at least a month after their most recent dose.
Many sports organizations have banned finasteride because it can be used to mask steroid abuse. Since 2005, finasteride has been on the World Anti-Doping Agency's list of banned substances. However, it was removed from the list in 2009. Notable athletes who used finasteride for hair loss and were banned from international competition include skeleton racer Zach Lund, bobsledder Sebastien Gattuso, footballer Romário and ice hockey goaltender José Théodore.
In December 2009, the Medicines and Healthcare products Regulatory Agency in the UK announced new drug safety advice on finasteride and the potential risk of male breast cancer. The agency concluded that, although overall incidence of male breast cancer in clinical trials for finasteride 5mg was not significantly increased, a higher risk of male breast cancer with finasteride use cannot be excluded. A warning on this risk will be included in the product information.
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