Does Phentermine Burn Fat or Just Shrink Your Appetite? 

If you were in your teens or older during the 90s, you might remember the rise—and subsequent fall—of Fen-Phen (1). The duo of two drugs, fenfluramine and phentermine, was widely prescribed for weight loss … until it was discovered that fenfluramine caused heart valve defects in as many as a third of users (2, 3). 

The FDA pulled Fen-Phen from the market in 1997, but phentermine, half of the ‘miracle drug’ combo, was still approved for weight loss. Recently, the drug has gotten buzzy again as newer weight-loss medications, such as Wegovy and Zepbound, continue to inch their way into daily conversations.  

For some people, phentermine is an attractive alternative to GLP-1 drugs. It’s a pill instead of an injectable. It’s also significantly cheaper. But as with other weight loss medications, there’s a bunch of misinformation about what phentermine is and what it can do to help you reach your goal weight.

About the Experts 

Christopher McGowan, M.D., is triple-board-certified in internal medicine, gastroenterology, and obesity medicine. He’s also a leader in endobariatrics, a new field of gastroenterology that targets the obesity epidemic without bariatric surgery. He is also the founder, CEO, and chief medical officer of True You Weight Loss.

Sue DeCotiis, M.D., is triple-board-certified in internal medicine, functional medicine, and obesity medicine, and based in New York City. She specializes in medical weight loss, peptide therapy treatments, and hormone replacement therapy. 

How Does Phentermine Work?

Phentermine is a stimulant appetite suppressant in the amphetamine family. The FDA considers it a controlled substance, meaning you need a prescription to get it. You need a BMI of 30 or more, or a BMI of 27 with weight-related comorbidities, to qualify for an Rx (4).

Phentermine targets a receptor in the hypothalamus, where the hunger centers of the brain reside, explains obesity medicine physician Christopher McGowan, M.D. Stimulation of this area causes the release of the neurotransmitters norepinephrine and epinephrine, which together work to decrease appetite. And it gets the job done. “Phentermine is effective for weight treatment, and compared to placebo, it’s significantly more effective,” McGowan says.  

More recently, phentermine was paired with another drug, topiramate, under the brand name Qsymia to treat obesity long-term (5, 6).

Topiramate is an anticonvulsant medication that’s used to treat seizures and migraines. Its exact mechanism related to weight loss isn’t fully understood, but it’s thought to reduce the hunger hormone leptin and prevent the body from storing excess fat. It may also stimulate the production of brown fat, the type that scorches calories and regulates fat metabolism (7, 8).

Together, phentermine and topiramate rev weight loss by targeting different areas of the brain, McGowan says. “In combination, the weight loss is nearly twice the amount of phentermine alone,” he adds.

Phentermine for Weight Loss

Because phentermine curbs appetite, people taking it feel less hungry and typically don’t eat as much. 

While the drug certainly works, it’s unlikely you’ll drop major pounds. The average weight loss on phentermine is 5 percent of your total body weight. When phentermine is paired with topiramate as Qsymia, weight loss may be as great as 8 to 11 percent of total body weight. In comparison, the newer anti-obesity injectables are in the 15-20 percent range.

Phentermine, by itself, is only approved to use for up to 12 weeks (9), McGowan says. That’s because phentermine is a member of the amphetamine family, and experts have raised concerns about it potentially being an addictive substance. But, that’s been debunked when treating obesity (10). In fact, one 2019 study found that people using phentermine (by itself) for longer than 12 weeks lose more weight than those who use it short term (11). (This research also notes that it was safe to use past the 12-week mark.)

Obesity medicine physician Sue DeCotiis, M.D., says that some physicians are OK with prescribing it for long-term use, especially if they complete one three-month cycle on the drug and then take a break for a few months before starting again. 

But, both McGowan and DeCotiis warn of phentermine’s diminishing returns. “It has a tachyphylaxis effect, meaning that it stops working after a time,” DeCotiis says. There are a couple of reasons why phentermine stops working, however the most common is that you have maxed out the drug’s potential.  

“The effects can be short-lived, but short-term can curtail the appetite,” DeCotis notes. Losing just 5 percent of your body weight can have a positive impact on your health. Research has found that losing between 5 and 10 percent of body weight can help improve HDL (good) cholesterol levels and lower blood pressure levels, which can help reduce heart disease risk (12). However, if you’re looking to lose more, you may have to pursue other treatments, McGowan says. 

DeCotiis adds that if you struggle with portion control, phentermine may be a good fit. But if incessant food noises or slow metabolism is behind weight gain, you’d probably benefit more from a GLP-1.

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Burning Fat vs. Suppressing Appetite 

While you may have heard that phentermine is a fat burner, the truth is, that’s not entirely accurate. “There’s some evidence to suggest that phentermine may have a modest impact on metabolic rate, but not as significant as its impact on appetite,” McGowan says.

That said, phentermine may target fat loss indirectly: the drug can help you cut back on calories and give you a boost of energy to exercise. 

Older research found the phentermine and topiramate combo decreased waist circumference and improved blood glucose levels in otherwise healthy white women. This research also found that this combo delayed the progression of type 2 diabetes, a metabolic disorder linked to obesity (13).

Is Phentermine a Stimulant?

Phentermine stimulates the release of norepinephrine and epinephrine, which reduces appetite while also providing a surge of energy. That added pep in your step may push you to hit the gym or track to burn calories.

Of course, those stimulant properties can cause some notable side effects. Norepinephrine and epinephrine are also responsible for your fight-or-flight response (14). The side effects of the drug are essentially the same ones you’d experience if you were being chased by a lion or stressed about a presentation at work: anxiety, insomnia, increased heart rate, jitteriness, dry mouth, and the oh-so-fun gastrointestinal upset (15).

“Generally speaking, I wouldn’t recommend phentermine for someone who has an anxiety disorder or significant anxiety that isn’t well controlled,” McGowan says, “because this is only going to amplify it.”

Also, avoid combining phentermine with other stimulants. “Reducing caffeine intake is a good idea,” McGowan says. “Other medications for ADHD or any other stimulant-type medications can be problematic, so you don’t want to combine those.”

DeCotiis also cautions about phentermine’s effect on the heart. “You certainly would not want anybody with a history of seizures to take it,” she says. Someone with an arrhythmia or a rapid heart rate would not be a good candidate for this drug, she explains. 

How To Burn Fat on Phentermine

As with any weight-loss program or medication, phentermine works best when combined with diet and exercise, say the experts. 

Focus on healthy foods

Because you’re eating fewer calories while taking phentermine, it’s important to prioritize nutrient-dense foods to meet your nutritional needs. 

“While taking phentermine, you’re going to be less hungry,” McGowan says. “Hopefully you’ll be eating smaller portions. So you really need to optimize [the nutritional quality of] those portions.” He suggests focusing on protein, as it’s satiating, and balancing the rest of your plate with whole foods as much as possible. DeCotiis echoes this: She recommends reducing carbs and increasing protein, fibrous veggies, and healthy fats.

McGowan recommends consulting with a dietitian when beginning phentermine or any weight-loss medication. 

Get in the (right) zone with exercise

This is going to seem counterintuitive, but, if you go hard on the treadmill all the time, and you’re not seeing results, DeCotiis says that’s your sign to dial back on cardio. 

“When you’re doing high-intensity cardio, your tissues need more sugar for fuel,” she says. “Insulin is the delivery system that delivers sugar to the tissues. If it’s doing that, it can’t burn fat.” 

Balance those tough workouts with moderate-intensity zone 2 workouts (brisk walking, jogging, biking, and swimming) that burn fat as a source of energy, DeCotiis says. The fat-burning zone is generally considered to be about 50–70 percent of your maximum heart rate. To calculate your max heart rate, subtract your age from 220. For example, if you’re 40, your max heart rate is 180 beats per minute and 90–126 bpm would be your fat-burning range (16).

And don’t skip the heavy weights. You’ve heard it before: Muscle burns fat. DeCotiis recommends lifting weights or doing resistance workouts that use your body weight, like Pilates, plyometrics, or working out with resistance bands. 

Consider a smart scale

Body Mass Index (BMI)—which all old-school scales can help you assess—doesn’t give you a comprehensive picture of your health. DeCotiis has her patients use body-composition scales, regardless of the diet or weight-loss program they’re following. These scales send tiny (and painless) electrical currents through the feet to determine your fat, water, and muscle composition. 

They aren’t cheap and the science on their accuracy is mixed, but DeCotiis says they can give a more accurate picture of the type of weight you’re losing. “If someone loses 10 pounds, [the question is] did they lose 10 pounds of water, muscle or fat,” she says. “Without that information, we’re just guessing.”

The Bottom Line

Phentermine is a safe, effective, and FDA-approved appetite suppressant designed for short-term use and smaller weight loss goals. Taken alone, it can lead to about 5 percent reduction in body weight. Combined with topiramate it can help you lose 8 to 11 percent of your body weight.