Phentermine / Topiramate ER
A daily capsule that suppresses appetite through two different brain pathways
Brand names: Qsymia
Quick read · 5 min
People who tend to consider this:
- ✓Prefer a daily pill and want to avoid injections
- ✓Want one of the most affordable prescription options
- ✓Are comfortable with its side effect profile
This may not be right if:
- ✕You are pregnant or could become pregnant (causes birth defects)
- ✕You have glaucoma or hyperthyroidism
- ✕You have a history of kidney stones
- ✕You want to avoid a controlled substance
Not a complete list. Whether this treatment is right for you depends on factors only your doctor can assess.
Weight loss — what trials show
In the main 56-week trial at the highest dose, people lost an average of about 9.8% of their body weight — roughly 22 lbs for a 220 lb person. This was about 8% more than people taking a dummy pill. About 66% of participants lost 5% or more of their body weight on the full dose.
These are averages from clinical trials — individual results vary. Trials compare the drug against a dummy pill, not against other weight loss drugs.
See the full trial data →How this compares
Average weight loss in clinical trials. Not a ranking — individual results vary.
Averages from clinical trials at highest approved dose. Not everyone achieves these results.
What to expect — week by week
The phentermine component typically reduces appetite within the first week.
Most people notice measurable weight loss by week 4–8.
Appetite suppression typically begins within the first week; significant weight loss by 12–16 weeks
Most weight loss has occurred by this point.
What happens if you stop?
Weight regain after stopping is expected. No large dedicated withdrawal study published. As with all anti-obesity drugs, obesity is a chronic condition requiring ongoing management.
Weight trajectory — during treatment and after stopping
Pattern estimated — no large randomised withdrawal trial published for this drug.
Most of the lost weight returns within a year of stopping — this is the body returning to its defended weight, not a failure. Long-term use or maintenance therapy can help preserve results.
Side effects
Frequencies from CONQUER. Most stomach and gut side effects are mild-to-moderate and peak during the dose increase period (first 12–16 weeks).
Percentages from CONQUER. Source: trial publication.
- •Do not take this drug if you are pregnant or could become pregnant — topiramate causes serious birth defects (cleft palate). Two forms of contraception are required. A monthly pregnancy test is part of the FDA safety programme.
- •This drug can increase heart rate. Your doctor should monitor pulse at each visit.
- •Topiramate can cause metabolic acidosis (a build-up of acid in the blood) and kidney stones with prolonged use.
This is not a complete list. Your doctor can advise based on your full medical history.
Muscle and lean mass
No large published body composition substudy is available for phentermine/topiramate specifically. As a primarily appetite-suppressing drug without the metabolic effects of GLP-1 agonists, lean mass preservation depends heavily on protein intake and resistance training during the calorie deficit.
What does the lost weight consist of?
Resistance training (weight lifting, bodyweight exercises) can significantly reduce the lean mass portion. Aim for 2–3 sessions per week during weight loss.
Proportions estimated — detailed body composition data not available from published trials for this drug.
Cost & access
~$30–$80/month generic; ~$200–$400/month brand (Qsymia)
Requires a prescription from a licensed healthcare provider. Prices vary. Ask your doctor about savings programs.
~$30–$400/moper month (list price)
Generic version (available 2025): ~$30–$80/month. Brand Qsymia: ~$150–$400/month.
How to take it
- •Take one capsule every morning — do not take in the afternoon or evening (may cause insomnia)
- •Swallow the capsule whole — do not crush, chew, or break it
- •Your doctor may increase your dose after 12 weeks if needed
- •Do not stop suddenly — taper the dose gradually under medical supervision
How it works
Phentermine suppresses appetite by triggering the release of adrenaline-like chemicals in the brain's hunger control centre. Topiramate's weight loss mechanism is not fully understood but likely involves reduced appetite, changes in how food tastes, and increased feelings of fullness.
Cardiovascular & other benefits
In CONQUER, significant reductions in blood pressure, waist circumference, triglycerides, and blood glucose were observed. However, phentermine raises heart rate — careful monitoring is required in people with cardiovascular disease.
- ✓Generic now available (from 2025) — significantly lower cost than GLP-1 drugs
- ✓Improved blood pressure and triglycerides in CONQUER trial
- ✓Oral daily pill — no injection required
- ✓Fastest appetite suppression onset of any approved drug
Community insights
These are personal experiences shared in public online communities — not medical advice.
“The 'brain fog' side effect (topiramate) is real for some people — it can affect word-finding and short-term memory. Discuss with your doctor if this becomes an issue.”
“Now that generic is available, cost is dramatically lower. Some pharmacies charge under $50/month.”
“Drink plenty of water — topiramate increases kidney stone risk, and staying well hydrated is one of the main ways to reduce it.”
Common questions
After reading this page, most people compare this with other medications, check if they qualify, or learn about side effects in more detail.
Next step most people take
Based on clinical trials · No rankings · Every claim linked to source
Last reviewed: March 2026