Qsymia contains between 3.75 mg and 15 mg of phentermine, depending on the dosage strength prescribed. That’s significantly less than the 37.5 mg found in standalone phentermine (sold as Adipex-P), because Qsymia pairs a lower dose of phentermine with a second ingredient, topiramate, to achieve weight loss through two different pathways.
Phentermine in Each Qsymia Strength
Qsymia comes in four capsule strengths. Each contains a fixed combination of phentermine and extended-release topiramate:
- Qsymia 3.75/23: 3.75 mg phentermine + 23 mg topiramate
- Qsymia 7.5/46: 7.5 mg phentermine + 46 mg topiramate
- Qsymia 11.25/69: 11.25 mg phentermine + 69 mg topiramate
- Qsymia 15/92: 15 mg phentermine + 92 mg topiramate
The first number in each name always refers to the phentermine content, the second to the topiramate. You take one capsule per day, in the morning.
How the Doses Are Used
Not all four strengths are used in the same way. The lowest dose (3.75/23) is a starter dose, typically taken for the first 14 days to let your body adjust. After that, most people move up to the 7.5/46 strength, which is the standard maintenance dose.
If you haven’t lost at least 3% of your body weight after 12 weeks on the 7.5/46 dose, your prescriber may increase you to the higher strengths. The 11.25/69 capsule serves as another transitional step, taken for 14 days before moving to the maximum dose of 15/92. So in practice, you’ll spend most of your time on either 7.5 mg or 15 mg of phentermine per day.
Why Qsymia Uses Less Phentermine Than Adipex
Standalone phentermine is prescribed at up to 37.5 mg per day. Qsymia’s maximum of 15 mg is less than half that amount. The reason is the addition of topiramate, which contributes its own appetite-suppressing effects through a completely different mechanism. Phentermine works by triggering the release of norepinephrine in the brain’s appetite center, which reduces hunger. Topiramate suppresses appetite and increases feelings of fullness through activity on a separate set of brain signaling pathways.
Because these two drugs attack the problem from different angles, lower doses of each can produce meaningful weight loss while reducing the side effects you’d get from a full dose of either one alone. Phentermine at higher doses tends to raise heart rate and blood pressure more noticeably, so keeping the dose lower is a practical advantage.
What the Lower Phentermine Dose Means for Side Effects
The stimulant-related side effects of phentermine, like insomnia, dry mouth, increased heart rate, and jitteriness, are dose-dependent. At 7.5 mg or even 15 mg, these effects are generally milder than what people experience on 37.5 mg of standalone phentermine. That said, topiramate brings its own set of potential side effects, including tingling in the hands and feet, changes in taste, difficulty concentrating, and constipation.
One serious concern specific to Qsymia is pregnancy risk. Topiramate can cause birth defects. The FDA requires a monthly pregnancy test for anyone who could become pregnant while taking the medication, and effective contraception is required throughout treatment.
How Qsymia Compares to Taking Phentermine Alone
If you’re comparing Qsymia to a standalone phentermine prescription, the key tradeoff is straightforward: Qsymia gives you less phentermine but adds a second active ingredient. The combination approach tends to produce greater weight loss than either drug alone at its respective dose. It also allows for longer-term use. Standalone phentermine is typically limited to 12 weeks of use, while Qsymia is approved for ongoing treatment as long as it remains effective and well-tolerated.
The phentermine in Qsymia is the same compound found in Adipex-P and generic phentermine tablets. It is not modified or reformulated. The topiramate component, however, is in an extended-release form, which means it dissolves gradually throughout the day rather than all at once.