Merck's Propecia is used to treat hair loss, but the drug can also cause a number of side effects, including permanent sexual dysfunction.

In 1997, pharmaceutical giant Merck & Co. introduced Propecia as a new use for finasteride, a synthetic compound the company developed five years earlier. Originally marketed as Proscar, doctors first prescribed 5 mg of finasteride to treat benign prostatic hyperplasia (BPH), more commonly known as an enlarged prostate gland.

Merck later discovered a remarkable side effect of Proscar treatment — some patients started to grow hair. By dropping the dose of finasteride to 1 mg, researchers found a way to correct male pattern baldness, a hereditary condition that affects close to 50 percent of the male population 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, approximately 400,000 men in the United States were filling prescriptions for Propecia and taking the drug daily. Although Merck provided the early Propecia and Proscar users with a list of sexual side effects researchers uncovered in clinical trials, 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. What it failed to mention, however, was 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.


Propecia and Proscar belong to a class of drugs called 5-alpha-reductase inhibitors, which are used to treat conditions stimulated by dihydrotestosterone (DHT), a potent hormone the body creates from testosterone. DHT-induced conditions include androgenic alopecia (male pattern baldness), benign prostatic hyperplasia (enlarged prostate) and prostate cancer.

In people genetically predisposed to male pattern baldness, 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. Nearly 35 million men in the United States suffer from this type of hair loss.

With Propecia, the active ingredient finasteride causes a rapid decline in DHT concentration to combat male pattern baldness. Within 24 hours of taking Propecia, the concentration of DHT drops by 65 percent. The full effects of Propecia can take three months or more to appear.


Propecia Side Effects

The most common side effects associated with Propecia include:

  • Decreased libido
  • Erectile dysfunction
  • Ejaculation disorder

The most frequently reported adverse reactions to Propecia affect men’s sexual health. A U.S. Food and Drug Administration (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.

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 others 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 Male breast cancer

It is worth mentioning that no clear causal links between Propecia use and sexual side effectshave been established. The drug’s method of action, however, makes the connection highly plausible. Finasteride, the active ingredient in Propecia, suppresses the body’s level of DHT, an androgen hormone that plays a major role in sexuality and sexual development.

Prostate Cancer

In June 2011, the FDA released a safety announcement warning that Propecia and other 5-alpha-reductase inhibitors may increase the risk for a serious form of prostate cancer. Known as high-grade prostate cancer, this is the most deadly kind.

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.

FDA Warnings

When clinical trial results 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.

In 2011, the FDA added that some adverse events, such as erectile dysfunction, may continue after the drug is no longer being used. And on April 11, 2012, the FDA made another revision, explaining that some patients continued to experience sexual side effects after cessation of use.

Side effects listed by the FDA include:
Libido disorders Ejaculation disorders
Orgasm disorders Decreased libido
Poor semen quality Infertility

To come to this conclusion, 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, with events including erectile dysfunction, decreased libido, ejaculation issues and orgasm disorders.


Proscar, like Propecia, is a 5-alpha-reductase inhibitor drug containing the active ingredient finasteride. Used clinically to treat benign prostatic hyperplasia (enlarged prostate) and help manage the condition’s urinary symptoms, 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 approximately 200 times the breast cancer rate of the general population.

Propecia Lawsuits

While the sexual side effects and cancer risks associated with Propecia use only affect a small percentage of users, these problems can take a drastic emotional toll on patients. Persistent sexual dysfunction, even after the end of treatment, can destroy personal relationships and diminish quality of life.

Hundreds of men have decided to take legal action against Merck, claiming the company omitted important information about long-term side effects from Propecia’s safety label. These men hope to receive compensation for the damaging effects Propecia caused.

By April 2012, there were Propecia lawsuits against Merck pending in six states, all related to persistent sexual dysfunction. To conserve resources, simplify the discovery process and keep pretrial rulings consistent, these cases were centralized to the Eastern District of New York on April 16, 2012. These cases were consolidated into MDL 2331, a multidistrict litigation led by U.S. District Judge John Gleeson. As of January 2013, the MDL had grown to 139 lawsuits. Another 250 cases are pending in New Jersey state courts.








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