Phentermine For Weight Loss: Uses, Dosage And Side Effects (2024)

Phentermine is a weight loss medication that belongs to a class of drugs called anorectics, or appetite suppressants. Approved by the U.S. Food and Drug Administration (FDA) for the treatment of obesity, phentermine is one of the oldest prescription medications on the market for weight loss, says Stephanie O. Page, D.O., a family medicine physician and medical director for the Metabolic and Medical Weight Loss Center at Nuvance Health in Wilton, Connecticut.

Typically, phentermine is prescribed for people with a BMI over 30 or a BMI over 27 with at least one weight-related condition, such as high blood pressure, high cholesterol, type 2 diabetes or sleep apnea. Because there’s limited research on its long-term effects on health, it’s only approved for short-term use of three months or less and should also be combined with changes to your diet and lifestyle. In some cases, health care professionals may also prescribe phentermine for longer periods or prescribe it intermittently to prevent weight regain.

Phentermine is available in generic form and is sold under brands like Adipex-P, Lomaira and Suprenza. It’s also sometimes paired with topiramate, a type of anticonvulsant that is approved for long-term weight management in adults with obesity.

Phentermine is sometimes confused with Phen-Fen (a combination of phentermine and fenfluramine), a weight loss co*cktailthat was heavily prescribed in the 1990s but was removed from the market due to increased rates of valvular disease, notes Dr. Page. While fenfluramine was the cause of the noted side effects, “phentermine alone has been clinically studied and has [not] been shown to increase risk of cardiovascular disease,” she says.

Phentermine suppresses hunger by blocking the reuptake of certain substances in the brain, including norepinephrine, serotonin and dopamine (which are involved in appetite and the sensation of feeling full), according to Kirsten Fredericksen, M.D., a Reno-based obesity medical specialist and Director of Clinical Operations at FormHealth, an online weight loss clinic.

“[Phentermine] targets the center in the brain that controls appetite,” adds Dr. Page, explaining that it can also reduce cravings and help people make better nutrition choices, which could promote weight loss.

Interestingly, a 2019 study comparing the effectiveness of various anti-obesity medications found that phentermine resulted in the highest amount of weight loss over a 12-week period, with an average weight loss of 8.3 pounds. Phentermine combined with topiramate came in close second, resulting in an average weight loss of 8 pounds after 12 weeks. However, it’s important to keep in mind that the authors of this study received funding from several pharmaceutical companies^[2]Shibuya K, et al. The Benefit of Short-Term Weight Loss with Anti-Obesity Medications in Real World Clinical Practice. Endocr Pract. 2019;25(10):1022-1028. .

Dr. Fredericksen clarifies that phentermine is actually an amine (an organic derivative of ammonia) rather than an amphetamine, or stimulant, and is unlikely to cause dependence. “Although it is a Schedule IV controlled substance, it is not considered habit-forming,” she explains.

The recommended dosage for phentermine ranges from 3.75 milligrams to 37.5 milligrams per day, according to Dr. Fredericksen. “Usually, doses are given as 8 milligrams, three times per day, 30 minutes before meals, or 15 to 37.5 milligrams once per day, before breakfast or one to two hours after breakfast.”

If prescribed phentermine, a doctor can provide guidance on the most appropriate dosage for you, depending on your needs.

Like other weight loss medications, phentermine is not approved for long-term use, and many doctors will recommend it to be cycled on and off.

Phentermine is not recommended for certain groups, including people who are pregnant or breastfeeding, those with a history of medical conditions like heart disease, glaucoma, hyperthyroidism or seizures and individuals who have taken a type of antidepressant called monoamine oxidase inhibitors (MAOIs) within the last 14 days, notes Dr. Page.

There is also limited research on its long-term safety and effectiveness, as phentermine is typically only indicated for short-term use of three months or less.

Phentermine can cause several possible side effects, according to Dr. Page, including:

• Dry mouth
• Insomnia
• Headache
• Increased blood pressure
• Increased heart rate
• Anxiety
• Constipation
• Metallic taste in the mouth

“This does not happen to all patients, and working with an obesity medicine provider in figuring out the best medication for you is key,” she says.

Anti-obesity medications, such as phentermine, are just one piece of the puzzle when it comes to weight management. “Prescription medications do work best when combined with a healthy nutrition plan, exercise and lifestyle changes,” says Dr. Page, adding that these medications alter physiology, but not behavior.

“In addition to the possible use of prescription anti-obesity medication, it’s important to address healthy nutrition, adequate physical activity and include habit-forming goals and behavioral support,” notes Dr. Fredericksen. She recommends working with health care providers who are experienced in weight management to help you reach your goals and maintain weight loss long-term.

Here are a few healthy habits that can help you get started, according to the Centers for Disease Control and Prevention (CDC):

• Keep a food diary to identify emotional triggers that can lead to overeating.
• Eat more slowly and learn to recognize when you’re feeling satisfied.
• Plan out a few nutritious, well-rounded meals ahead of time.
• Aim for 150 minutes of moderate-intensity physical activity each week, or 30 minutes five days per week.
• Find activities that you enjoy doing and build it into your routine.
• Exercise with friends or family members to stay motivated.