Weight Loss & Metabolic Health
What Is Phentermine for Weight Loss?
Last reviewed: May 2026 · Haute MD Editorial Team
Phentermine is an oral appetite suppressant in the sympathomimetic amine class, FDA-approved since 1959 for short-term weight loss in adults with a BMI ≥30, or ≥27 with comorbidities. It stimulates the release of norepinephrine in the brain to reduce hunger and produces average weight loss of 5–10% over 3–6 months. It is the most prescribed weight-loss medication in the United States outside the GLP-1 class.
How phentermine works
Phentermine increases the release of norepinephrine (and to a lesser extent dopamine and serotonin) in the central nervous system, suppressing appetite at the hypothalamus and increasing alertness. The effect is similar in mechanism to amphetamines, though phentermine has lower abuse potential and is classified as a Schedule IV controlled substance. Typical dosing is 15–37.5 mg once daily in the morning, with effects on appetite usually felt within the first few days.
Effectiveness and combination use
On its own, phentermine produces an average weight loss of 5–8% over 12 weeks. Combined with topiramate (as Qsymia), weight loss increases to 9–11%. Many physicians use phentermine off-label for longer than the FDA-approved 12 weeks under close monitoring when patients are tolerating it well and continuing to benefit. Phentermine is markedly less expensive than GLP-1 medications, often $20–40 per month, making it accessible to patients without GLP-1 coverage.
Side effects, contraindications, and monitoring
Common side effects include increased heart rate, elevated blood pressure, insomnia, dry mouth, constipation, and anxiety. It is contraindicated in patients with cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, history of substance abuse, pregnancy, or use of MAO inhibitors. Blood pressure and heart rate should be checked at baseline and during treatment. It is not appropriate for long-term passive use without lifestyle changes and physician follow-up.
Frequently Asked Questions
How is phentermine different from a GLP-1?
Phentermine acts on the central nervous system to suppress appetite and is oral, cheap, and fast-acting. GLP-1s work on multiple hormonal pathways, produce greater and more durable weight loss, and are injectable and expensive.
Can I take phentermine long-term?
FDA-approved for short-term use, but many physicians prescribe longer under monitoring. Long-term safety data are limited compared to newer agents.
Will phentermine work after I stop it?
Like other weight-loss medications, regain is common after discontinuation unless lifestyle changes are maintained.
Can I combine phentermine with a GLP-1?
Some physicians use combinations cautiously in specific cases. This should only be done under specialist supervision.
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