Phentermine May Help You Lose Weight, but Not Without Risks

If you have a BMI of 30 or higher, your doctor may suggest trying a weight-loss medication, like phentermine.
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If you're trying to lose weight through diet and exercise, but not seeing the results you want, you may be wondering about taking weight-loss medications. One well-known option is the weight-loss pill phentermine, but is it safe?


When used properly, phentermine helps with weight loss, but when it is given in too high doses or to the wrong person, it can be dangerous for your heart and kidneys, explains Scott Kahan, MD, MPH, the director of the National Center for Weight and Wellness in Washington, D.C.

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What Does Phentermine Do?

Phentermine (sold under brand names like Adipex, Ionamin and Suprenza) is approved for short-term use only. It is an appetite suppressant and a stimulant, according to the Obesity Medicine Association. Phentermine is also available in combination with topiramate for weight loss under the brand name Qsymia.


Your doctor may consider weight-loss medications such as phentermine if your body mass index (BMI) is 30 or more, which is considered obesity. (BMI is a measure of your weight in relation to your height.) If you have a BMI of 27 or more and other weight-related health concerns such as type 2 diabetes, you may also be a candidate for phentermine, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

"Phentermine tends to lower appetite, and not everyone who [has] overweight or obesity has a high appetite," Dr. Kahan says. "Some people don't feel full or have hard-to-control cravings as opposed to having an overactive appetite." Dr. Kahan says he would consider phentermine for someone with a large appetite.


Another advantage? The low cost, he says. Phentermine is less expensive than other weight-loss medications. It is also sometimes combined with another medication.

"You only get moderate weight loss with phentermine by itself, but when it is combined with topiramate, we see much more weight loss and this weight loss is durable," Dr. Kahan says. "With 3 months, we aim for 10 to 15 pounds of weight loss with phentermine alone, but sometimes we will get a lot more," he says.


Although phentermine is approved only for short-term use, some doctors may use it for longer periods of time. "Long-term use of phentermine may produce more weight loss," Dr. Kahan says. But it is not a quick fix and must be used in combination with healthy eating and regular exercise, he notes.


Phentermine Side Effects and Dangers

Phentermine side effects include dry mouth, insomnia, dizziness and irritability, according to the Mayo Clinic.


Phentermine gets a bad rap because it was part of the popular weight-loss pill combo known years ago as Fen-Phen. The fenfluramine part was pulled from the market in 1997 after it was linked to potentially fatal heart valve problems. Phentermine was deemed safe but is still guilty by association in some people's minds, Dr. Kahan says.

Phentermine side effects tend to occur when the medication is given in doses that are too high, Dr. Kahan says. "It is a mild stimulant, but at high doses, it can be very stimulating. You many feel anxious, your heart rate will increase and you won't sleep well."


Phentermine may also increase blood pressure and is not recommended if you have severe or uncontrolled high blood pressure, he says. It is also not for people with kidney disease because high blood pressure can worsen kidney function, per the National Kidney Foundation. Phentermine also isn't a good option if you have heart disease, an overactive thyroid gland or glaucoma, according to the Cleveland Clinic.

Safe Use of Phentermine

The key to safe use is that your doctor needs to make sure it is right for you and prescribe the lowest effective dose, Dr. Kahan says. "The maximum U.S. Food and Drug Administration-approved dose is 37.5 milligrams per day, but in most cases, much lower doses work quite well."


Tell your doctor about all of the other medications you take, as some may interact with phentermine, including certain antidepressants known as MAO inhibitors and other diet medications, according to the American College of Cardiology. If you are pregnant, plan to become pregnant or are breastfeeding, phentermine is not right for you, per the Mayo Clinic.

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