Short answer: Phentermine/Topiramate has a half-life of about 20 hours (phentermine); about 65 hours (topiramate). Levels peak 6 hours (phentermine); 9 hours (topiramate) after a dose. People vary, so treat these as estimates.
Phentermine/topiramate is a once-daily capsule that combines phentermine, a stimulant appetite suppressant, with extended-release topiramate, a seizure medicine. It is sold as Qsymia for long-term weight management with diet and exercise.
Also known as: Qsymia, phentermine-topiramate ER.
Qsymia comes in four strengths (phentermine/topiramate): 3.75 mg/23 mg, 7.5 mg/46 mg, 11.25 mg/69 mg and 15 mg/92 mg.
The Qsymia label approves it, with a reduced-calorie diet and more activity, to reduce excess weight and keep it off long term in adults and children 12 and older with obesity, and in adults with overweight and at least one weight-related condition. Label dosing: 3.75 mg/23 mg each morning for 14 days, then 7.5 mg/46 mg. After 12 weeks, adults who have lost less than 3% of body weight go to 11.25 mg/69 mg for 14 days, then 15 mg/92 mg. If weight loss is under 5% after 12 weeks on 15 mg/92 mg, Qsymia is stopped, tapering over at least a week, because stopping topiramate suddenly can cause seizures. Evening doses are avoided because of insomnia. The label says phentermine is a Schedule IV controlled substance and topiramate is not controlled.
Qsymia has no boxed warning, but it can cause birth defects (cleft lip or palate), so a negative pregnancy test is recommended before starting and every month during treatment, and it is sold only through certified pharmacies under a REMS program. It is not used in pregnancy, with glaucoma or an overactive thyroid, or within 14 days of an MAO inhibitor. Other warnings include suicidal thoughts, sudden eye problems, mood, sleep and thinking problems, metabolic acidosis, kidney stones and less sweating. In 1-year trials, tingling (4% to 20% vs 2%), dry mouth (7% to 19% vs 3%), constipation, taste change, insomnia and dizziness rose with dose. See Qsymia side effects, Qsymia dosing schedule, missed Qsymia dose, how to store Qsymia, and Lomaira vs Qsymia, Contrave vs Qsymia, Qsymia vs Wegovy, Saxenda vs Qsymia and Xenical vs Qsymia.
The Qsymia label says that after a single 15 mg/92 mg capsule, phentermine peaked at 6 hours and topiramate at 9 hours; a high-fat meal did not change either. The mean terminal half-life is about 20 hours for phentermine and about 65 hours for topiramate, so both build up with daily dosing (about 2.5-fold and 4-fold). Most of each leaves unchanged in the urine.
Numbers shown are taken from the source above. Half-life varies from person to person.
How Dose Track handles peptides · Reconstitution calculator · Half-life calculator