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What Is Phentermine?

Phentermine is a prescription appetite suppressant used for short-term weight management in people who meet certain obesity-related criteria. It belongs to the sympathomimetic amine class and is prescribed alongside a reduced-calorie diet, physical activity, and behavioral changes. Phentermine is available in oral tablets and capsules under several generic and brand-name products, including Adipex-P. It is classified as a Schedule IV controlled substance in the United States. Phentermine is not intended for indefinite treatment, and its stimulant-like effects require careful assessment of cardiovascular health and other risk factors.

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What Is Phentermine?

Phentermine is a prescription appetite suppressant used for short-term weight management in people who meet certain obesity-related criteria. It belongs to the sympathomimetic amine class and is prescribed alongside a reduced-calorie diet, physical activity, and behavioral changes. Phentermine is available in oral tablets and capsules under several generic and brand-name products, including Adipex-P. It is classified as a Schedule IV controlled substance in the United States. Phentermine is not intended for indefinite treatment, and its stimulant-like effects require careful assessment of cardiovascular health and other risk factors.

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How Does Phentermine Work?

Phentermine acts primarily on the central nervous system to reduce appetite. Its sympathomimetic activity is associated with increased signaling by norepinephrine and other neurotransmitters involved in hunger regulation. By decreasing appetite, phentermine can help patients follow a reduced-calorie eating plan. Its exact mechanism in weight reduction has not been fully established. Phentermine does not directly block dietary fat absorption or correct the underlying causes of obesity. Tolerance to its appetite-suppressing effects can develop, and increasing the dose beyond the prescription is not recommended.

Who Can Receive Phentermine for Weight Management?

Phentermine is indicated as a short-term adjunct to weight reduction in patients with an initial body mass index of at least 30, or at least 27 when additional risk factors such as controlled hypertension, diabetes, or elevated cholesterol are present. It is generally prescribed to adults, and certain formulations are not recommended for patients aged 16 years or younger. Phentermine should not be used during pregnancy or breastfeeding. It is also contraindicated in people with cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, certain agitated states, or a history of drug abuse.

How Should Phentermine Be Taken?

Phentermine dosing depends on the prescribed formulation and individual response. For common 37.5 mg phentermine hydrochloride tablets, the usual adult dose is one tablet daily before breakfast or one to two hours afterward. Some patients receive a lower dose or divided doses according to prescribing instructions. Late-evening administration should be avoided because it can cause insomnia. Patients with severe kidney impairment may require a reduced maximum daily dose. Phentermine is intended for short-term use, generally a few weeks, and should not be combined with other weight-loss medications without medical direction.

What Are the Side Effects and Warnings of Phentermine?

Common adverse effects include dry mouth, insomnia, restlessness, dizziness, constipation, and increased heart rate. Phentermine can raise blood pressure and may cause palpitations or other cardiovascular symptoms. Rare but serious concerns include pulmonary hypertension and heart valve disease. It also carries risks of misuse, dependence, and impaired alertness. Phentermine must not be taken with monoamine oxidase inhibitors or within 14 days of their use. New chest pain, unexplained breathlessness, fainting, or swelling of the legs requires prompt medical evaluation.

Common Questions About Phentermine

Is phentermine a stimulant?

Phentermine is a sympathomimetic appetite suppressant with stimulant-like effects on the central nervous system. It can increase alertness, heart rate, and blood pressure.

How long is phentermine prescribed?

Phentermine is approved for short-term weight management, generally for a few weeks, alongside dietary changes and physical activity.

Can phentermine cause dry eyes or blurred vision?

Phentermine can cause dry mouth and visual disturbances in some patients. It is contraindicated in people with glaucoma, and new or significant visual symptoms require medical evaluation.

Is phentermine a controlled substance?

Yes. Phentermine is a Schedule IV controlled substance because it has potential for misuse and dependence.

References

DailyMed - PHENTERMINE HYDROCHLORIDE tablet. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0ee5b71b-3d8e-46d0-937d-2dbe43490e33. Date Accessed October 30, 2026.

Phentermine. LiverTox. https://www.ncbi.nlm.nih.gov/books/NBK547916/. Date Accessed October 30, 2026.

Phentermine. MedlinePlus. https://medlineplus.gov/druginfo/meds/a682187.html. Date Accessed October 30, 2026.

Phentermine. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/Phentermine. Date Accessed October 30, 2026.

Phentermine Capsules or Tablets. Cleveland Clinic. https://my.clevelandclinic.org/health/drugs/18539-phentermine-capsules-or-tablets. Date Accessed October 30, 2026.