Most people taking phentermine and topiramate (sold as Qsymia) lose between 9% and 10.5% of their body weight over one to two years. For someone starting at 250 pounds, that translates to roughly 23 to 26 pounds. More than half of people on the medication lose at least 10% of their starting weight, compared to fewer than 12% of people making lifestyle changes alone.
What Clinical Trials Show
The most robust long-term data comes from a two-year trial called SEQUEL, which tracked patients on two different doses. Those on the standard dose (7.5 mg phentermine/46 mg topiramate) lost an average of 9.3% of their body weight by week 108. Those on the higher dose (15 mg/92 mg) lost 10.5%. The placebo group, which followed the same diet and exercise recommendations, lost just 1.8%.
A more recent trial paired the medication with an intensive lifestyle program that included a 1,200 to 1,400 calorie daily diet, 10,000 daily steps, and 150 minutes of weekly exercise. At 12 months, the phentermine-topiramate group lost an average of 33.7 pounds, while the lifestyle-only group lost 12.9 pounds. Even at the three-month mark, the medication group had already lost about 23.8 pounds versus 8.9 pounds with lifestyle changes alone. These results suggest the combination of medication plus structured diet and exercise produces considerably more weight loss than either approach on its own.
How the Two Drugs Work Together
Phentermine is a stimulant-type appetite suppressant. It reduces hunger signals in the brain, making it easier to eat less without feeling like you’re fighting constant cravings. Topiramate, originally developed as a seizure medication, also suppresses appetite but through different pathways. It appears to change how your brain perceives fullness and may influence hormones and brain chemicals involved in food reward and energy balance.
The combination works because the two drugs target appetite through separate mechanisms, producing a greater effect together than either would alone. This also means each drug can be used at a lower dose than it would need to be on its own, which helps reduce side effects.
How the Dosage Ramps Up
You don’t start at the full dose. The medication begins at a low “starter” strength (3.75 mg/23 mg) for 14 days, then increases to the standard dose (7.5 mg/46 mg). You stay at that level for 12 weeks while your progress is evaluated.
If you haven’t lost at least 3% of your body weight after those 12 weeks, the dose gets bumped up again through a brief two-week transition before reaching the maximum strength (15 mg/92 mg). If you still haven’t lost at least 5% of your body weight after another 12 weeks at the highest dose, the medication should be discontinued. It’s not effective for everyone, and there’s a built-in checkpoint to make sure you’re actually benefiting before continuing.
Who Can Take It
The FDA approved phentermine-topiramate for adults with a BMI of 30 or higher, or a BMI of 27 or higher if they also have at least one weight-related condition such as high blood pressure, type 2 diabetes, or high cholesterol. It’s meant to be used alongside a reduced-calorie diet and regular physical activity, not as a standalone fix.
Timeline for Results
Weight loss typically starts within the first few weeks, and the three-month mark is the first major milestone. In the intensive lifestyle trial, participants had already lost a significant amount by month three. The SEQUEL trial showed that losses continued through the first year and were largely maintained through year two, with no significant rebound while still on the medication. This is notable because many weight-loss treatments show diminishing returns after six months.
Your individual results will depend on your starting weight, the dose you end up on, how closely you follow a calorie-controlled diet, and your activity level. The clinical trials required participants to make real dietary and exercise changes alongside the medication, so the weight loss numbers reflect that combined effort.
Common Side Effects
The most frequently reported side effects include tingling in the hands and feet (a well-known effect of topiramate), dry mouth, constipation, altered taste, and insomnia. Many of these are mild and tend to improve over the first few weeks as your body adjusts. The tingling sensation, called paresthesia, is usually harmless but can be uncomfortable.
Because phentermine is a stimulant, some people notice an increased heart rate or feel jittery, particularly at higher doses. Topiramate can occasionally cause difficulty concentrating or word-finding problems, sometimes called “brain fog.”
Pregnancy Risk
One serious safety concern applies to women who could become pregnant. Topiramate is associated with a two- to five-fold increased risk of oral clefts (cleft lip or cleft palate) in babies exposed during the first trimester. Because of this, the FDA requires a negative pregnancy test before starting the medication and monthly pregnancy testing throughout treatment. Effective contraception is essential the entire time you’re taking it, and the medication must be stopped immediately if pregnancy occurs.
What Realistic Results Look Like
If you’re starting at 200 pounds and respond well to the standard dose, expect to lose roughly 18 to 21 pounds over the first year. At 300 pounds, that range shifts to about 28 to 32 pounds. Adding a structured diet and exercise plan can push results higher, as the intensive lifestyle trial showed losses averaging over 33 pounds at one year.
Keep in mind that “average” means some people lose more and some lose less. Over half of participants in clinical trials hit the 10% threshold, which is the amount of weight loss associated with meaningful improvements in blood pressure, blood sugar, and cholesterol levels. But the built-in 12-week evaluation exists for a reason: if the medication isn’t producing results for you, continuing it is unlikely to change that, and stopping allows you and your provider to explore other options.