Phentermine, a generic drug that has been around for decades, has managed to hold its own despite the arrival of new competitors.

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The Food and Drug Administration (FDA) has approved several new weight-loss drugs in recent years, but the best-selling diet pill in America isn’t among them.

That title belongs to phentermine, a generic drug that has been around for decades and has managed to hold its own despite the arrival of new competitors.

The drug is viewed as effective and relatively safe to help jump-start diets in patients who are obese. However, phentermine — a stimulant that can give users an inexpensive high — has a long history of misuse. It has also frequently flown under the radar of regulators, who tend to focus their resources on deadlier drugs, such as opioid painkillers.

Adding to the concern is that many doctors have an incentive to recommend it: Phentermine is a mainstay of weight-loss clinics nationwide, prescribed by physicians who profit by selling the pill to patients. Fans of the drug offer tips on social-media sites and websites like, where users refer to each other as “phrends” and share the names of doctors who will prescribe it with few questions asked.

“There’s just not enough resources to shut down everybody that should be shut down,” said Carmen Catizone, executive director of the National Association of Boards of Pharmacy.

Phentermine’s staying power has persisted despite FDA approval of four new weight-loss products since 2012, all of which arrived with great fanfare but whose sales have failed to live up to expectations.

Phentermine — approved in 1959 and now made by several manufacturers — commands 80 percent of the market for diet drugs, according to IMS Health, which tracks prescription-drug use.

Phentermine is inexpensive, often about $30 for a month’s supply. The newer drugs, by contrast, can cost hundreds of dollars a month and are sometimes not covered by insurance.

In the 1990s, phentermine was used as one-half of the combination treatment known as fen-phen, the weight-loss sensation that was later discovered to cause heart-valve problems in some patients. The other two drugs commonly used in the treatment, fenfluramine and dexfenfluramine, were withdrawn from the market. Phentermine, which was not linked to the heart problem, was allowed to remain available.

Because phentermine was approved when drug companies weren’t held to the same standards as new drugs, little is known about the drug through formal trials.

Still, patients who take phentermine must be screened because the drug can intensify existing heart problems and it can be abused by people with eating disorders. Users often develop a tolerance and require higher doses. It is approved only for short-term use in obese patients, in conjunction with a plan that includes exercise and a healthy diet.

But many doctors prescribe phentermine to patients for long-term use and say the practice is safe. “It’s lamentable that it’s not being used more widely, because it really does work,” said Dr. Ed Hendricks, who runs the Hendricks for Health weight-loss clinic in Sacramento, Calif. He has conducted research on phentermine use and concluded it is not addictive.

Others said phentermine could be misused. “It’s sort of like a cheap speed,” said C. Richard Allen, director of the Georgia Drugs and Narcotics Agency, which oversees the use of controlled substances such as phentermine.

Judi Wade, a former phentermine user in New York, credits the drug with helping her reduce her weight to 120 pounds from 140 pounds.

Wade, 50 and unemployed, bought the drugs from a doctor who advertised in a newspaper. He charged $200 for 60 pills, she said, and offered discounts for buying in bulk.

Wade said the physician, Dr. Samir Mostafa, rarely weighed her or took her blood pressure. She said she stopped taking phentermine after about a year because she didn’t like the way it made her feel.

In 2013, New York state’s Board of Professional Medical Conduct stripped Mostafa of his ability to practice medicine, saying he had failed to properly report his dispensing activities and had been inappropriately prescribing the drug. Amy Kulb, a lawyer for Mostafa, declined to comment.

Among the state’s claims was that Mostafa had prescribed “excessive amounts” of phentermine to another patient, whose blood pressure he also failed to monitor. The woman, state regulators said, was 5-foot-7 and weighed 93 pounds.