Phentermine is an appetite suppressant available since the 1950s, and topiramate is a seizure medication that suppresses appetite in some people. A drug company called Vivus combined phentermine and topiramate into a single pill called Qsymia, and it was FDA-approved for weight loss in 2012. Below is a comparison of the two drugs.

• Phentermine dosing: Qsymia is available in four different strengths, with the strongest pill containing 15 mg of phentermine and 92 mg of extended-release topiramate per day. Phentermine by itself is typically dosed at 37.5 mg per day.
• Efficacy: In respective placebo-controlled trials, phentermine 30 mg once daily and the highest dose of Qsymia (15 mg/92 mg) both caused around 10% weight loss over 6 months to a year. In a trial comparing phentermine 15 mg once daily (less than half the standard dose) to Qsymia 15 mg/92 mg once daily, weight loss over 28 weeks was 6.1% in the phentermine group and 9.2% in the Qsymia group. Both drugs are FDA-approved for weight loss in adults, while Qsymia is also approved for weight loss in adolescents aged 12 years and older.
• Side effects: Common side effects of phentermine include dry mouth, insomnia, and jitteriness, while topiramate can cause paresthesias (tingling in the extremities), lightheadedness, taste perversion, and fatigue. Topiramate has also been associated with birth defects and should not be taken by pregnant women or those hoping to become pregnant.
• Cost:
• Phentermine: Phentermine is cheap, costing $12 to $30 for thirty tablets with GoodRx coupons.
• Qsymia: In May 2025, a generic version of Qsymia was approved, and a month's supply costs between $60 and $90 at most pharmacies. As additional generic manufacturers produce Qsymia, the price should continue to decline. The individual drugs can also be prescribed separately, and both have inexpensive generics.

Summary: Qsymia is effective, and some patients prefer it, while others do not like topiramate's side effects. In studies, phentermine 30 mg once daily has the same efficacy as the highest dose of Qsymia. For patients interested in Qsymia who have never taken phentermine, I typically recommend starting with phentermine. That way, they are only being exposed to one set of side effects. If weight loss is not progressing on phentermine alone, topiramate can always be added later. Patients who cannot tolerate phentermine's side effects may find Qsymia more tolerable because it contains lower doses of phentermine.

• Qsymia review
• Phentermine review