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Is phentermine for weight loss safe?

Maura Rhodes By | September 23, 2019
Medically reviewed by Anis Rehman, MD

Phentermine is a prescription diet pill that suppresses appetite. For people who are obese, or have a weight-related medical condition, this appetite suppressant can be a life-changing drug. Phentermine is  effective, safe to take short-term (three months), and inexpensive. For people who just want to fit into their skinny jeans, phentermine would never be medically advised or prescribed.

What is phentermine?

Phentermine is the oldest prescription weight-loss pill used today to treat obesity. It’s also the drug that’s most commonly prescribed for this purpose, even though there are newer options on the market.

First, it’s popular because it’s effective. Studies have found it can bring about a loss of 5% to 10% of body weight over 12 weeks, when taken as part of a treatment plan that includes a low-calorie diet and increased physical activity.

Second, it’s inexpensive, according to Caroline M. Apovian, MD, a professor at Boston University School of Medicine who specializes in endocrinology, diabetes, nutrition, and weight management. Many insurance companies won’t pay for weight loss medication or treatment, despite plenty of research confirming obesity’s health dangers. Phentermine may be the only viable option for people with limited income.

Phentermine is available under the brand names Adipex-P and Lomaira.

How does phentermine work for weight loss?

Phentermine is, in medical lingo, an anorectic, which simply means it curbs hunger. It does this, in part, by triggering the release of certain brain chemicals in the area of the hypothalamus that controls appetite, explains Karl Nadolsky, MD, a board-certified endocrinologist who specializes in diabetes, metabolism, and obesity at Spectrum Health in Grand Rapids, Michigan, and a Fellow of the American College of Endocrinologists (FACE).

Hence, phentermine helps decrease appetite by affecting the central nervous system.

How long should you take phentermine? 

Phentermine is only approved for short-term use. The long-term safety and effectiveness of phentermine has never been studied—nor will it be, according to Dr. Apovian, who explains that the type of rigorous research necessary would be far too expensive.

Phentermine side effects

Serious adverse effects are rare. The most common side effects of phentermine include:

  • Rapid heart rate
  • Tingling in the hands or feet
  • Dry mouth
  • Sleeplessness
  • Nervousness
  • Constipation

You should not take phentermine if you have heart disease  including valvular heart disease, heart failure or high blood pressure, depression, an overactive thyroid, or glaucoma, or are pregnant, trying to become pregnant, or breastfeeding.

Is phentermine safe?

Phentermine is an often maligned medication. The diet pill has a risky reputation for two main reasons:

1. Phentermine is an amphetamine-like medication. 

It’s chemically similar to amphetamines. So, the U.S. Food and Drug Administration (FDA) classifies it as a Schedule IV drug (i.e., a federally controlled substance), because of concerns for potential abuse or addiction. Dr. Nadolsky points out, phentermine is not an amphetamine. “Phentermine appears to be safe—even for long-term use,” he says. “And in my practice, I’ve never seen or heard of a problem with phentermine.”

2. Phentermine was half of the drug duo phen-fen.

In the 1990s, physicians prescribed phentermine with fenfluramine or dexfenfluramine, a pairing nicknamed phen-fen. Phen-fen became a miracle weight-loss craze, until the FDA in 1997 realized fenfluramine and dexfenfluramine were causing serious heart problems and pulled them from the market.

The bottom line

You should only take phentermine or weight loss if you are obese—with a body mass index (BMI) of over 30—or a BMI of over 27 if have another serious medical condition caused by obesity, like diabetes or hypertension.

The real danger of phentermine lies—not in the drug itself, but in how it may be mishandled. There are what Dr. Nadolsky refers to as “phentermine factories” that are not associated with a hospital or other bona fide medical programs. They sell phentermine directly to clients looking for fast, easy weight loss without screening for risk factors—a practice that is illegal and risky.

When prescribed as part of an overall treatment plan by a doctor who specializes in obesity or endocrinology and who will provide careful oversight, it’s unlikely phentermine will be problematic. It should always be part of a balanced program, paired with lifestyle changes such as:

  • a reduced-calorie, nutrient-dense diet
  • increased physical activity
  • behavioral changes

Without these basic foundations of healthy weight loss, you’re likely to gain back all of the pounds dropped after you stop taking phentermine.

With these keystones, and appropriate medical supervision, phentermine, can be highly effective as part of a short-term weight loss program.