Phentermine is a prescription appetite suppressant used for a limited period as part of a weight-management plan. It does not directly “burn” body fat. It mainly reduces appetite, which may help some people eat fewer calories.
How Phentermine Works
Phentermine is a stimulant-like medicine in a class called anorectics. It decreases appetite and is intended to be used alongside dietary changes and physical activity.
It is not a shortcut for rapid weight loss, and taking more than prescribed does not make it safer or more effective.
Who Needs Extra Caution?
Phentermine may not be appropriate for people with certain cardiovascular conditions, uncontrolled high blood pressure, hyperthyroidism, glaucoma, a history of drug misuse, or some other medical conditions. It must not be taken with monoamine oxidase inhibitors (MAOIs) or within the relevant washout period directed by a clinician.
If you have diabetes, weight loss and reduced food intake can change medication needs. Do not reduce insulin or other diabetes medicines on your own—your prescriber may need to adjust them.
Pregnancy and Breastfeeding
Phentermine is not appropriate for weight loss during pregnancy. If pregnancy occurs while taking it, contact the prescriber. Breastfeeding decisions should also be discussed with a healthcare professional.
Common and Serious Side Effects
Possible side effects include dry mouth, constipation or diarrhea, unpleasant taste, restlessness, insomnia, dizziness, tremor, and increased heart rate or blood pressure.
Seek medical care promptly for chest pain, shortness of breath, fainting, swelling of the legs or ankles, severe palpitations, or difficulty doing activity you could previously tolerate.
Dependence and Duration of Use
Phentermine can be habit-forming and should be taken only for the duration and dose prescribed. Do not increase the dose, use it more often, or continue it longer than instructed.
What About “Fat Burners”?
Over-the-counter “fat burners” are not a safer substitute. Many contain stimulants or multiple ingredients, and their effectiveness and safety may be uncertain. Combining stimulant supplements with phentermine may increase side effects.
Bottom Line
Phentermine mainly suppresses appetite; it does not selectively melt belly fat or directly burn fat cells. It can help selected patients when prescribed and monitored appropriately, but cardiovascular risks, interactions, pregnancy, diabetes medicines, and dependence all need consideration.
Reliable Source
Last reviewed for medical accuracy: September 2026.