Short answer: Phentermine has a half-life of about 20 hours (Qsymia label). Levels peak about 6 hours (in Qsymia) after a dose. People vary, so treat these as estimates.
Phentermine is a stimulant related to amphetamine, used for a few weeks as an appetite suppressant alongside diet and exercise. It is sold as Lomaira (8 mg tablets) and in other strengths such as Adipex-P, and combined with topiramate as Qsymia. This page uses the Lomaira label.
Also known as: Lomaira, Adipex-P, phentermine hydrochloride.
Lomaira is an 8 mg tablet of phentermine hydrochloride (6.4 mg phentermine base), scored so it can be split in half. Other phentermine products come in other strengths with their own labels.
The Lomaira label approves phentermine as a short-term (a few weeks) add-on to exercise, behavior change and fewer calories for obesity, in people with a starting BMI of 30 or more, or 27 or more with other risk factors such as controlled high blood pressure, diabetes or high cholesterol. Label dosing: one 8 mg tablet three times a day, half an hour before meals, at the lowest dose that works; late evening doses are avoided because of insomnia. The label says phentermine is a Schedule IV controlled substance. If the appetite effect wears off, the label says not to raise the dose but to stop.
The label has no boxed warning. It is not used with a history of heart or blood vessel disease, within 14 days of an MAO inhibitor, with an overactive thyroid, glaucoma, agitated states or a history of drug abuse, or in pregnancy or while nursing. Rare, often fatal pulmonary hypertension and heart valve disease have been reported with appetite suppressants, including with fenfluramine or dexfenfluramine, and rarely with phentermine alone. Side effects, listed without percentages, include fast heartbeat, raised blood pressure, restlessness, insomnia, dry mouth and constipation. See Lomaira side effects, Lomaira dosing schedule, Lomaira vs Qsymia, diethylpropion vs phentermine and phendimetrazine vs phentermine.
The Lomaira label gives no half-life or time to peak. The Qsymia label, which contains phentermine, gives a mean phentermine terminal half-life of about 20 hours and a time to peak of 6 hours after a single 15 mg/92 mg capsule; 70% to 80% of a dose leaves unchanged in the urine. The Lomaira label says exposure can rise with kidney impairment.
Numbers shown are taken from the source above. Half-life varies from person to person.
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