Trump's Hair Growth Medicine: What Is Finasteride?
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Donald Trump's personal doctor recently revealed that the president takes a handful of medications, including a daily aspirin to lower heart attack risk, an antibiotic to treat the skin condition rosacea, a statin to lower cholesterol — and a hair-loss drug called finasteride.

The physician, Dr. Harold Bornstein, credited finasteride with helping maintain Trump's head of hair, as well as his own long locks. "He has all his hair," Bornstein told The New York Times. "I have all my hair."

But what is finasteride, and how does it work?

The drug, which is also known by brand names such as Propecia, is an oral medication approved by the Food and Drug Administration (FDA) to treat and reverse male pattern baldness. Male pattern baldness, which typically describes hair loss that begins at the temples (also often called the "Hippocratic wreath"), can leave men with a bald top and a narrow ring of hair around the sides. [Macho Man: 10 Wild Facts About His Body]

Finasteride is a type of drug called a 5-alpha-reductase inhibitor, and it works by preventing the breakdown of testosterone into a byproduct called dihydrotestosterone (DHT). DHT can fuel male pattern baldness because it quiets another chemical signal that acts at the hair follicle and fuels new hair cell generation and growth, according to a 2015 review of studies in the journal Expert Opinion in Drug Discovery.

Finasteride's action on DHT may also explain why Trump's medical records show low levels of the protein PSA, or prostate-specific antigen. High levels of PSA can be a marker for prostate cancer. Low levels can indicate that a person has had treatment for prostate cancer or an enlarged prostate, according to The New York Times.

Finasteride has gone through three placebo-controlled trials (where people got either the drug or a sugar pill) to assess its effects on male pattern baldness. According to drug approval data from the FDA, men who had taken finasteride had 107 more hairs per square inch (16 per square centimeter) after one year of use and 277 more hairs per square inch (43 per square cm) after five years of use. The average person may have anywhere from 1,050 to 1,935 hairs per square inch (175 to 300 hairs per square centimeter), according to a 1965 article in the Journal of Investigative Dermatology.

Although finasteride may be effective in terms of statistics, whether the drug has a meaningful or obvious visual benefit may be in the eye of the beholder.

The drug has relatively few side effects, according to a 2016 review of studies in the Journal of Clinical of Aesthetic Dermatology. Between 3.4 percent and 15.8 percent of men using the product report sexual dysfunction, such as erectile dysfunction and decreased libido, and there was a slight association with depression. Men who take the drug may also experience a decrease in the volume of their ejaculate, according to the FDA approval data. The male fetuses of women who take the drug may also be at increased risk of birth defects.

One small study also found that finasteride reduced men's consumption of alcohol, though exactly why wasn't clear.

Originally published on Live Science.