Phentermine: Use, Side Effects, and Precautions

What is Phentermine, and What is it Used For?

Phentermine is a medication designed to suppress appetite, primarily used for the short-term treatment of obesity. It is recommended for individuals with a body mass index (BMI) equal to or greater than 30 kg/m2 or those with a BMI greater than or equal to 27 kg/m2, accompanied by risk factors for heart disease. Combining Phentermine with diet and exercise is essential for optimal results. It’s crucial to note that Phentermine should not be used in conjunction with other weight loss agents, and the combination of Phentermine and fenfluramine (FEN-PHEN) is associated with severe heart problems. Fenfluramine is no longer available in the United States.

What are the Side Effects of Phentermine?

Phentermine may induce various side effects, including nausea, vomiting, diarrhea, dry mouth, constipation, an unpleasant taste, hives, impotence, palpitations, high blood pressure, and fast heart rates (palpitations). Central nervous system side effects may include hyperactivity, insomnia, restlessness, tremors, and dizziness. Notably, the combination of Phentermine and fenfluramine has been linked to pulmonary hypertension and heart valve defects, presenting serious complications. Long-term use of Phentermine may lead to physical and psychological dependence, with withdrawal symptoms such as excessive drowsiness, fatigue, tremors, and depression after prolonged use.

What is the Dosage for Phentermine?

The recommended dose is 15-37.5 mg once daily, administered about 1-2 hours after breakfast. It’s advisable to use the lowest effective dose, avoiding evening doses due to the potential for insomnia.

Which Drugs Interact with Phentermine?

Phentermine, similar to amphetamine, may share interactions associated with amphetamines. Combining Phentermine with tricyclic antidepressants or monoamine oxidase inhibitors (MAOIs) can result in elevated serotonin levels and high blood pressure. Phentermine should not be used within 14 days after stopping an MAOI. Individuals with diabetes may require a reduction in insulin or oral hypoglycemic drug dosage when using Phentermine. Combining alcohol with Phentermine may lead to adverse reactions.

Pregnancy and Breastfeeding

The use of Phentermine during pregnancy has not been thoroughly evaluated, and its excretion in human breast milk is uncertain. Consequently, Phentermine is not recommended during breastfeeding, or if used, nursing should be discontinued.

What Else Should I Know About Phentermine?

Phentermine is available in tablet form (8 and 37.5 mg) and capsules (15, 30, 37.5 mg). It should be stored in a tight container at room temperature, between 20°C to 25°C (68°F to 77°F). The exact mechanism of action of Phentermine is unknown, but it is believed to decrease appetite by acting on brain centers that control hunger. As a stimulant similar to amphetamine, Phentermine stimulates the nervous system, raises blood pressure, and increases heart rate.


Phentermine is an appetite-suppressant medication used for short-term weight loss in conjunction with diet and exercise. It comes with potential side effects and precautions, and interactions with certain drugs should be carefully considered. Pregnant and breastfeeding individuals should exercise caution, and proper storage conditions must be maintained.

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