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Finasteride (brand name: Proscar) treats benign prostatic hyperplasia, a condition in which the prostate becomes abnormally enlarged 23. Information from the National Library of Medicine describes finasteride as a member of a group of drugs named for the enzyme they target, 5-alpha reductase 123. By blocking the activity of this particular enzyme, finasteride prevents it from producing hormones that enlarge the prostate 23. The drug does not cure the condition, however, and once the medication is stopped, the prostate may begin to enlarge once again. Although the five-milligram dose of finasteride is commonly prescribed for enlarged prostate, the drug is also available in a one-milligram dose to treat hair loss in men 23.
Rare side effects
The Mayo Clinic notes that rarely patients report having had side effects associated with finasteride 23. These side effects are not typically serious but if they worsen or continue, contact a physician. Rare side effects include headache, rhinitis (stuffy nose) and back pain. Also, some patients may experience abdominal pain and diarrhea. Patients may feel weak or dizzy. The dizziness results from postural hypotension, a side effect of finasteride that causes blood pressure to drop upon standing 23. If the condition worsens, fainting or seizures may occur so feelings of dizziness should be monitored closely.
Common side effects
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By far the most commonly reported side effect of finasteride deals with sexual function 23. According to Merck, disruption of sexual function remains the most frequent reason for stopping treatment during clinical studies. Some patients may experience decreased libido and sexual desire. Moreover, men may have difficulty achieving an erection and have less ejaculation, as well as pain in the testicles.
- By far the most commonly reported side effect of finasteride deals with sexual function 2.
- According to Merck, disruption of sexual function remains the most frequent reason for stopping treatment during clinical studies.
Severe side effects
Severe allergic reactions to finasteride require immediate medical attention 3. Effects related to drug allergy or hypersensitivity includes pruritis (itching, often related to hives), urticaria (hives), rash and swollen lips and face. Male patients who notice breast changes should consult with a physician. These changes could include an enlargement of the breasts, as well as the presence of lumps or liquid discharge from the nipples. Painful breasts may also indicate an adverse effect of the drug. Merck also notes that finasteride may cause peripheral edema--a swelling of the body’s extremities that usually occurs in the legs and feet 23.
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- Medline Plus: National Library of Medicine: Finasteride
- The Mayo Clinic: Finasteride
- Merck: Proscar (finasteride)
- NIH: National Cancer Institute. Definition of 5-alpha reductase inhibitor.
- NIH: National Institute of Diabetes and Digestive and Kidney Diseases. Prostate enlargement (benign prostatic hyperplasia). Updated September 2014.
- U.S. Food & Drug Administration. Highlights of prescribing information: Proscar (finasteride) tablets. Updated September 2013.
- U.S. National Library of Medicine: MedlinePlus. Finasteride. Updated January 15, 2018.
- U.S. Food & Drug Administration. Highlights of prescribing information: Propecia. Updated April 2012.
- Tartagni M, Schonauer MM, Cicinelli E, et al. Intermittent low-dose finasteride is as effective as daily administration for the treatment of hirsute women. Fertil Steril. 2004;82(3):752-5. doi:10.1016/j.fertnstert.2004.02.118
- Unger CA. Hormone therapy for transgender patients. Transl Androl Urol. 2016;5(6):877-884. doi:10.21037/tau.2016.09.04
- Unger JM, Hershman DL, Till C, et al. Using medicare claims to examine long-term prostate cancer risk of finasteride in the prostate cancer prevention trial. J Natl Cancer Inst. 2018;110(11):1208-1215. doi:10.1093/jnci/djy035
- U.S. Food &Drug Administration. FDA Drug Safety Communication: 5-alpha reductase inhibitors (5-ARIs) may increase the risk of a more serious form of prostate cancer. Updated June 9, 2011.
- National Cancer Institute. Prostate cancer prevention – for health professionals (PDQ).
- Thompson IM, Goodman PJ, Tangen CM, et al. The influence of finasteride on the development of prostate cancer. N Engl J Med. 2003;349(3):215-24. doi:10.1056/NEJMoa030660
- Goodman PJ, Tangen CM, Darke AK, et al. Long-term effects of finasteride on prostate cancer mortality. N Engl J Med. 2019;380(4):393–394. doi:10.1056/NEJMc1809961
- Azzouni F, Mohler J. Role of 5α-reductase inhibitors in prostate cancer prevention and treatment. Urology. 2012;79(6):1197–205. doi:10.1016/j.urology.2012.01.024
- Hoque A, Yao S, Till C, et al. Effect of finasteride on serum androstenedione and risk of prostate cancer within the prostate cancer prevention trial: differential effect on high- and low-grade disease. Urology. 2015;85(3):616–20. doi:10.1016/j.urology.2014.11.024
- Murtola TJ, Gurel B, Umbehr M, et al. Inflammation in benign prostate tissue and prostate cancer in the finasteride ARM of the prostate cancer prevention trial. Cancer Epidemiol Biomarkers Prev. 2016;25(3):463–9. doi:10.1158/1055-9965.EPI-15-0987
Based in New York City, Amber Angelle has been a science writer since 2008. Her articles have appeared in "Popular Mechanics," "Discover" and "Popular Science." Angelle also contributed to the textbook "Psychology Around Us" and to the encyclopedia series Salem Health: Cancer. She holds a Master of Science in pharmacology from Tulane University School of Medicine.