PharmaceuticalStimulantAnorectic

Phentermine

Also known as Adipex-P · Lomaira · Qsymia (with topiramate)

Written by enwyReviewed 15 Jul 20262 sources0 comments

An amphetamine-related sympathomimetic anorectic approved for short-term weight loss as an adjunct to diet and exercise. It suppresses appetite by increasing noradrenaline release. Widely used and studied, but indicated only for short courses because of tolerance, cardiovascular effects and dependence potential.

01 Overview

Phentermine is the most commonly prescribed weight-loss drug in the US. It is approved for short-term use (typically up to 12 weeks) in patients with obesity, and produces modest weight loss of a few kilograms beyond diet alone. It is also combined with topiramate in the extended-use product Qsymia.

Because it is structurally an amphetamine, it raises heart rate and blood pressure and can cause insomnia, dry mouth and, rarely, dependence. It is Schedule IV in the US — lower abuse potential than classic amphetamines but not negligible. Historical association of the fen-phen combination with valvular heart disease was attributed to fenfluramine, not phentermine.

02 Mechanism

Stimulates release of noradrenaline (and to a lesser extent dopamine) in the hypothalamus, reducing appetite via central sympathetic signalling.

03 Dosing

Tier	Dose	Route	Notes
Therapeutic (short-term weight loss)	15–37.5 mg/day	Oral	Once daily before or after breakfast; short-term use, typically up to 12 weeks.

04 Effects

Effect	Magnitude	Evidence
Appetite suppression and weight lossProduces modest short-term weight loss as an adjunct to lifestyle change.	~3-5 kg beyond diet over 12 weeks	Clinical

05 Side effects

Effect	Severity	Frequency	Evidence
InsomniaDifficulty sleeping, especially with later dosing.	Mild	Common	Clinical
Increased heart rate and blood pressureSympathomimetic elevation of heart rate and blood pressure; contraindicated in uncontrolled hypertension.	Moderate	Common	Clinical
Tolerance and dependenceAnorectic effect wanes with time and dependence can occur, hence short-term indication.	Moderate	Uncommon	Clinical

06 Legal status

United States

Schedule IV controlled

as of Jan 2026

07 References

Phentermine and topiramate extended-release for obesity (CONQUER): a randomised, placebo-controlled trialThe Lancet, 2011

Adipex-P (phentermine hydrochloride) Prescribing InformationUS FDA

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.