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Propecia is a men’s only prescription drug used for the treatment of male pattern hair loss (androgenetic alopecia). Propecia works by binding the male hormone DHT to receptors in hair follicles at the top of the scalp.

Propecia Pills
Propecia Facts
  1. Dosage 1 mg tablet taken once daily
  2. Used to Treat Male-pattern hair loss (androgenetic alopecia)
  3. Related Drugs Proscar; Avodart (dutasteride)
  4. Manufacturer Merck & Co. Inc.
  5. Active Ingredient Finasteride
  6. Treatment Duration Full effects of daily Propecia use can take three months or more to appear; stopping treatment leads to reversal of effect within 12 months
  7. FDA Approval Status Initially approved in 1992
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Propecia is a once-a-day pill treatment for male-pattern hair loss, or androgenetic alopecia. Male-pattern hair loss is when the hairline recedes and hair thins at the temples and crown. This condition is hereditary and affects close to 50 percent of the male population aged 50 years or older. It is sometimes recognized by a horseshoe-shaped fringe around the sides and the back of the head.

Propecia’s effects on hair loss were actually discovered by accident. Originally marketed as Proscar, doctors first prescribed 5 mg of finasteride (the active ingredient in Propecia) to treat benign prostatic hyperplasia (BPH), more commonly known as an enlarged prostate gland.

Merck later stumbled upon a remarkable side effect of Proscar treatment: some balding patients started to grow hair back. By dropping the dose of finasteride to 1 mg, researchers found a way to correct male-pattern hair loss.

In 1997, Merck introduced Propecia as a new use for finasteride. The company had developed the synthetic compound five years earlier.

Middle Aged Man with Hair Loss
Male-pattern hair loss affects almost 50 percent of males aged 50 or older

With nearly half a billion dollars of research behind it, Propecia entered the drug market to high acclaim. After its first year, about 400,000 men in the U.S. were filling prescriptions for Propecia and taking the drug daily. Merck provided early Propecia users with a list of sexual side effects researchers had uncovered in clinical trials. However, it later came to light that the company wasn’t telling the whole story.

Upon approval of the drug, Merck disclosed that 3.8 percent of Propecia patients had one or more adverse sexual experiences, compared with 2.1 percent of patients taking a placebo. The company failed to warn, however, that these problems can persist even after patients stop taking the drug. In fact, Merck originally claimed that all of these issues would resolve after discontinuation of the drug.

The U.S. Food and Drug Administration (FDA) has since required updates to the drug’s label to reflect the potential for persistent sexual side effects with Propecia use. Thousands of men have sued Merck over allegations that the drug maker failed to provide adequate warnings and even “knowingly and recklessly omitted and concealed” known risks.

How Propecia Works

Baldness is caused by a potent male hormone called dihydrotestosterone, or DHT, which is produced in the adrenal glands, hair follicles, testicles and prostate gland. DHT is derived from the male sex hormone, testosterone. The enzyme 5α -reductase converts testosterone to DHT.

In people genetically predisposed to male-pattern baldness (androgenic alopecia), DHT binds to receptors in hair follicles at the top of the scalp. When the DHT hormone acts on these follicles, they shrink and gradually lose the ability to grow thick and healthy hair. As a result, the person’s hairline thins and recedes. Hair follicles on the back and side of the head are genetically resistant to DHT, which explains the common pattern seen in male baldness.

Propecia blocks the enzyme that converts testosterone to DHT, causing a rapid decline in DHT concentration.

Nearly 35 million men in the U.S. suffer from this type of hair loss. Other DHT-induced conditions include enlarged prostate and prostate cancer.

Propecia belongs to a class of drugs called 5-alpha-reductase inhibitors, which are used to treat conditions stimulated by DHT. Finasteride, the active ingredient in Propecia, blocks the enzyme that converts testosterone to DHT and causes a rapid decline in DHT concentration.

Propecia comes in the form of a tan, octagonal, film-coated tablet with a “P” logo on one side and “PROPECIA” embossed on the other. The drug’s label instructs users to take one 1 mg tablet once daily. Men can take Propecia with or without meals.

Within 24 hours of taking Propecia, the concentration of DHT drops by 65 percent. The full effects of daily Propecia use can take three months or more to appear. Stopping treatment leads to reversal of effect within 12 months, according to the drug’s label.

Propecia Side Effects

The most frequently reported adverse reactions to Propecia affect men’s sexual health. Although no clear causal links between Propecia use and sexual side effects have been established, finasteride suppresses the body’s level of DHT, which plays a major role in sexuality and sexual development, making the connection highly plausible.

An FDA analysis of adverse event data revealed a wide range of sexual side effects in otherwise healthy Propecia users ages 21 to 46. These problems persisted for an average of 40 months after the men ceased treatment.

The most common side effects associated with Propecia include:
  • Decreased libido
  • Erectile dysfunction
  • Ejaculation disorder

Adverse sexual events are most notable in younger men. Several side effects have been found to improve or completely resolve once Propecia treatment is stopped, while in others it may continue to affect the patient for several months or even years. In some cases, sexual function never returns to normal.

Other side effects of Propecia include allergic reactions, including rash, itching, hives and swelling of the lips and face; breast enlargement and tenderness; depression; testicular pain; male infertility or poor quality of semen; and male breast cancer.

FDA Propecia Warnings

When evidence conflicted with Merck’s original assertion that the sexual side effects of Propecia would stop once treatment ends, the FDA revised the drug’s safety label accordingly.

The FDA reviewed 421 post-marketing reports of sexual side effects sent to its Adverse Events Reporting Systems (AERS) database from 1998 to 2011. A total of 59 cases in the AERS database reported sexual dysfunction that continued for at least three months after Propecia use ended.

In 2011, the FDA updated the drug’s label to include information about incidences of male breast cancer and a warning that erectile dysfunction may continue after the drug is no longer being used.

On April 11, 2012, the FDA made another revision, explaining that some patients continued to experience libido disorders, ejaculation disorders and orgasm disorders after they stopped taking the drug. The agency also added a description of reports of male infertility and/or poor semen quality that improved or returned to normal after drug discontinuation.

Women, Children & Pregnancy

Pregnant women should not handle crushed or broken Propecia tablets because of possible risk to male fetuses.

The FDA warns that Propecia is not approved for use in women and children. Women should not handle crushed or broken Propecia tablets when they are pregnant or when they may potentially be pregnant because of potential risk to a male fetus.

DHT is a hormone necessary for normal development of male genitalia. As a result, if a pregnant woman receives finasteride, the drug may cause abnormalities of the external genitalia of a male fetus, according to the FDA.

In female rats, low doses of finasteride administered during pregnancy have produced abnormalities of the external genitalia in male offspring, including decreased prostatic and seminal vesicular weights, delayed preputial separation and transient nipple development.

Prostate Cancer

The drug also carries a warning that men 55 and older who take Propecia may have an increased risk of an aggressive type of prostate cancer. The warning is based on the dose to treat enlarged prostate, which is five times higher than the dose used for hair loss.

In June 2011, the FDA released a safety announcement warning that Propecia and other 5-alpha-reductase inhibitors may increase the risk for high-grade prostate cancer, the deadliest form of prostate cancer.

Prostate Cancer Scan
Men 55 and older who take Propecia may have an increased risk of an aggressive type of prostate cancer

High-grade prostate cancer is highly aggressive and grows rapidly, often spreading to other areas like the lymph nodes and bones. Further, high-grade prostate cancer cells are large, difficult to treat and reappear more frequently than low- and intermediate-grade prostate cancers.

Although the cancer risk appears to be low, the FDA distributed the announcement to inform doctors and patients of safety revisions to the warning and precaution labels for this class of drugs. Evidence of the increased risk comes from two large clinical trials: the seven-year Prostate Cancer Prevention Trial (PCPT) and the four-year Reduction by Dutasteride of Prostate Cancer Events (REDUCE) trial.

Both trials sought to determine if finasteride could be used to lower the risk of prostate cancer. While PCPT and REDUCE did in fact result in decreased incidences of lower-risk forms of prostate cancer, patients in both trials were found to be at a greater risk for high-grade prostate cancer.

Propecia is not the only 5α-reductase inhibitor currently available in the U.S. Two others, Proscar and Avodart, are also designed to prevent the conversion of testosterone to DHT. However, unlike Propecia, Proscar and Avodart are not indicated to treat male-pattern baldness. They are approved to treat BPH in men with enlarged prostate and to help manage the condition’s urinary symptoms.


Proscar Tablets
Proscar also uses finasteride – but contains five times the dose of Propecia

Proscar, like Propecia, is a 5-alpha-reductase inhibitor drug containing the active ingredient finasteride. Merck manufactures Proscar. The FDA approved the drug in 1992.

Proscar contains five times the dose of finasteride used in Propecia and offers a similar range of potential side effects.

In a four-year, placebo-controlled clinical study called the Proscar Long-Term Efficacy and Safety Study (PLESS), 3,040 patients between the ages of 45 and 78 with symptomatic BPH and an enlarged prostate were evaluated for safety. Of these patients, 3.7 percent treated with Proscar and 2.1 percent given a placebo discontinued treatment because of adverse reactions to sexual function.

In another study of patients taking Proscar alone or in combination with doxazosin (another 5-alpha-reductase inhibitor), four out of 1,554 patients developed breast cancer. This rate is about 200 times the breast cancer rate of the general population.


Avodart Tablet
Unlike Propecia, Avodart uses dusateride as its active ingredient

Though it has a different active ingredient than Propecia and Proscar, Avodart is also designed to block the enzyme that converts testosterone to DHT.

Like finasteride, dutasteride, the active ingredient in Avodart, is also associated with decreased libido, decreased amount of semen released during sex, impotence, and breast tenderness or enlargement. Avodart may also increase patients’ risk of developing prostate cancer.

The FDA first approved Avodart in 2001. GlaxoSmithKline manufactures the drug, which is available in once-daily 0.5 mg soft gelatin capsules.

While finasteride has no known drug interactions, dutasteride can interact with certain HIV drugs, such as ritonavir; certain blood pressure and angina drugs, such as amlodipine; and certain antibiotics, such as azithromycin. Taking dutasteride at the same time as any of these drugs may slow the breakdown of dutasteride in the liver, causing more of the drug to stay in the body.

As with Propecia, women and children should not take Avodart. The drug can cause birth defects if a woman is exposed to it during pregnancy.

Please seek the advice of a medical professional before making health care decisions.

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9 Cited Research Articles

  1. Grady, D. (2017, February 2).Trump Takes the Hair-Growth Drug Propecia. How Does It Work? Retrieved from
  2. (2014, January). Highlights of Prescribing Information. Retrieved from
  3. National Institutes of Health. (2013, January 17). Androgenetic Alopecia. Retrieved from
  4. U.S. Food and Drug Administration. (2012, April 11). Questions and Answers: Finasteride Label Changes.
  5. NY Daily News. (2012, April 16). Propecia and Proscar May Have More Sexual Side Effects than Labels are Telling. Retrieved from
  6. Merck & Co., Inc. (2012, April). Propecia Highlights of Prescribing Information. Retrieved from
  7. Traish, A. et al. (2010, December 22). Adverse Side Effects of 5-alpha-Reductase Inhibitors Therapy: Persistent Diminished Libido and Erectile Dysfunction and Depression in a Subset of Patients. Retrieved from
  8. Institute for Safe Medication Practices. (2013, January 9). Finasteride (Propecia, Proscar) and Possibly Persistent Sexual Side Effects Methylphenidate Pate (Daytrana) and Product Problems Update on Anticoagulants Dabigatran (Pradaxa) and Rivaroxaban (Xarelto). Retrieved from
  9. Hair Loss Learning Center. (n.d.). Propecia – Hair Loss Medication. Retrieved from
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