The revelation by his longtime doctor that President Trump takes a medication to prevent hair loss has piqued curiosity about the drug.
In an interview with The New York Times, the physician, Dr. Harold N. Bornstein, said that Mr. Trump takes finasteride, also marketed as Propecia.
The drug, a one-a-day pill, is a popular treatment for so-called male-pattern hair loss, in which the hairline recedes and hair thins at the temples and crown, sometimes to the point of leaving just a horseshoe-shaped fringe around the sides and the back of the head.
The culprit in this type of baldness is a male hormone, dihydrotestosterone or DHT. In some men, hair follicles are particularly sensitive to it, probably based on genetics. DHT forms when an enzyme converts testosterone into it; finasteride blocks the enzyme, lowering DHT levels.
The drug is better at stopping hair loss than at bringing back what’s already gone.
“I set expectations from the beginning, realistic expectations” about the drug, said Dr. Andrew Alexis, the chairman of the dermatology department at Mount Sinai St. Luke’s and Mount Sinai West in New York. “The thing I emphasize the most is stopping progression and preserving the hair you have. I think it’s very successful at doing that. “
Dr. Alexis is not involved with Mr. Trump’s care.
Most men with this type of hair loss start treatment with a different drug, minoxidil, also sold as Rogaine, which is rubbed into the scalp, Dr. Alexis said. If they do not see enough of an effect, they can add finasteride. Many of his patients wind up using both drugs, Dr. Alexis said. The treatment works best on the top of the head, and not as well at the temples and the receding hairline.
For both drugs, the effects on hair loss were discovered by accident, when patients were taking them for other reasons. Minoxidil, in pill form, was used to treat high blood pressure, and finasteride was prescribed for prostate enlargement. But balding patients noticed that some hair grew back.
Finasteride can have side effects, including decreased sex drive, erectile problems and a decreased volume of ejaculate. But those reactions are not common, affecting less than 2 percent of the men in studies.
Dr. Alexis said he warns patients about the potential problems, and that some men decline the drug as a result. But many decide to try it, and they generally stick with it, he said.
The drug also carries a warning that men 55 and over who take it may have an increased risk of an aggressive type of prostate cancer. But the warning is based on studies of a dose five times higher than the dose used for hair loss. The higher dose is used to treat enlarged prostates.
The risk, if any, for men taking the smaller dose is not known, Dr. Alexis said.