Phentermine vs. GLP-1s (and Qsymia, Contrave, Orlistat): How They Compare

Phentermine and the other older weight-loss medicines (Qsymia, Contrave and orlistat) typically lead to about 5% to 10% weight loss. That’s less than today’s GLP-1 medicines, where average trial results run from about 15% on Wegovy to about 21% on Zepbound. The older drugs cost less and still have a place, but each comes with trade-offs worth knowing.

Phentermine

Phentermine is a stimulant that suppresses appetite. It’s a controlled substance because it can lead to dependence and misuse, and it can raise blood pressure and heart rate, so it’s generally avoided in people with heart disease. It’s meant for short-term use, typically no more than about three months. Expect roughly 5% to 10% weight loss.

Qsymia (phentermine plus topiramate)

Qsymia combines phentermine with topiramate, a seizure and migraine medicine whose side effects include weight loss. Expect up to about 9% to 10% at the higher doses.

Contrave (bupropion plus naltrexone)

Contrave pairs bupropion, an antidepressant, with naltrexone, which is used for opioid and alcohol dependence. Together they reduce appetite and cravings. Side effects can include headache, constipation, trouble sleeping and dry mouth. Expect about 5% to 9% with diet and exercise.

Orlistat (Alli, Xenical)

Orlistat blocks the absorption of some of the fat you eat. It’s sold over the counter as Alli and at a higher dose by prescription. Its biggest drawback is oily, loose stools. Expect up to about 5%.

How they compare with GLP-1s

Medicine	Typical weight loss
Orlistat	up to about 5%
Contrave	about 5–9%
Phentermine	about 5–10% (short-term use)
Qsymia	up to about 9–10%
Wegovy (semaglutide)	about 15% in its main trial
Zepbound (tirzepatide)	about 21% in its main trial

These numbers come from separate trials and aren’t head-to-head comparisons. Which medicine fits depends on your health history, your goals, side effects and cost, which is a decision to make with your doctor.

Sources

• Prescribing information for phentermine (Adipex-P), Qsymia, Contrave and Xenical: indications, short-term use, cautions and trial results
• Wegovy prescribing information and Zepbound prescribing information: trial results