Most people taking phentermine and topiramate at the highest dose lose between 10% and 14% of their body weight over about a year. For someone starting at 250 pounds, that translates to roughly 25 to 36 pounds. The actual amount varies based on your dose, starting weight, and how consistently you pair the medication with diet and exercise changes.
Weight Loss by Dose and Timeframe
The clearest picture of what to expect comes from two large clinical trials, EQUIP and CONQUER, each lasting 56 weeks. In the EQUIP trial, which enrolled people with a BMI above 35, those on the maximum dose lost an average of 14.4% of their body weight. About two-thirds lost at least 5%, and nearly half lost 10% or more. The CONQUER trial, which included people with slightly lower BMIs (27 to 45), found an average loss of 12.4% at the top dose, with similar proportions hitting those 5% and 10% benchmarks.
Dose matters significantly. The combination comes in three strengths, and weight loss scales with each step up. People on the mid-range dose in these trials typically lost around 8% to 10% of their body weight, while those on the lowest starting dose lost considerably less. Your prescriber will start you at the lowest strength and increase gradually based on how you respond.
A two-year extension study called SEQUEL tracked people who stayed on the medication for 108 weeks. Those on the top dose maintained a 10.5% loss from their starting weight, while the mid-dose group held at 9.3%. The placebo group lost only about 2%. This is notable because weight regain is common with obesity medications, and the combination held its ground over two full years.
How the Medication Works
Phentermine and topiramate attack appetite through two different pathways, which is why the combination tends to outperform either drug alone. Phentermine triggers the release of a stress hormone called norepinephrine in the part of the brain that regulates hunger. This suppresses appetite and slightly increases the number of calories you burn at rest. Topiramate, originally developed for seizures, enhances calming signals in the brain while blocking excitatory ones. The net effect is reduced food cravings and a stronger sense of fullness after eating.
The combination lets each drug be used at lower doses than it would need on its own, which helps reduce side effects while keeping the appetite-suppressing benefits strong.
The 12-Week Checkpoint
Not everyone responds equally. About a third of people prescribed this type of medication lose less than 3% of their body weight in the first three months. These early non-responders are unlikely to see meaningful results even if they continue, so the prescribing guidelines build in a formal evaluation at 12 weeks. If you haven’t lost at least 3% of your starting weight on the mid-range dose by that point, your prescriber will either increase your dose or discuss stopping the medication. The same checkpoint applies again 12 weeks after any dose increase.
People who do respond early, losing 3% or more in those first three months, tend to have significantly better long-term outcomes. Early weight loss is the strongest predictor of where you’ll end up a year later.
How Dosing Works
Treatment begins with two weeks at the lowest strength to let your body adjust. After that, you move up to the mid-range dose, which you stay on for at least 12 weeks. If results are insufficient at that point and you’re tolerating the medication well, your prescriber can step you up through a brief transitional dose before reaching the maximum strength. Each increase comes with another 12-week evaluation window. The medication is taken once daily in the morning, since taking it later in the day can interfere with sleep.
Common Side Effects
The most frequently reported side effects are tingling or numbness (usually in the hands and feet), dry mouth, altered taste, insomnia, constipation, and dizziness. These tend to be more common at higher doses. In clinical trials, tingling affected about 20% of people on the top dose compared to 2% on placebo. Dry mouth hit 19% at the highest dose. Altered taste and insomnia each affected roughly 9%.
Most of these side effects are mild to moderate and often improve over the first few weeks. The tingling sensation, caused by topiramate, is the one that surprises people most. It’s harmless but can be annoying. Staying well-hydrated helps with both dry mouth and constipation.
Who Can Take It
The combination is prescribed for adults with a BMI of 30 or higher, or a BMI of 27 or higher if you also have a weight-related condition like high blood pressure, type 2 diabetes, or high cholesterol. It is not safe during pregnancy. Topiramate is associated with a two- to five-fold increase in the risk of cleft lip or cleft palate when taken during the first trimester. Because of this, the FDA requires a negative pregnancy test before starting and monthly testing throughout treatment. Effective contraception is required for anyone who could become pregnant.
What It Costs
The brand-name version, Qsymia, currently runs about $89 for a 30-day supply or $210 for a 90-day supply ($70 per month) through the manufacturer’s cash-pay program. If you have commercial insurance that covers it, you’ll typically pay a $70 copay with potential savings on subsequent refills. Insurance coverage for weight loss medications varies widely by plan, so checking your formulary before starting is worth the effort.
Realistic Expectations
A 10% to 14% reduction in body weight may sound modest on paper, but it’s enough to produce meaningful health improvements. Losing that amount typically lowers blood pressure, improves blood sugar control, reduces strain on joints, and decreases the risk of developing type 2 diabetes. The medication works best as part of a broader approach that includes dietary changes and increased physical activity. It reduces the appetite signals that make those changes so difficult to sustain, but it doesn’t replace them.
Weight loss with phentermine and topiramate tends to plateau around 9 to 12 months, after which the goal shifts to maintaining what you’ve lost. The two-year data showing sustained losses of around 10% suggest the combination is effective for maintenance as long as you continue taking it. Stopping the medication often leads to some regain, which is true of all current obesity medications.