Old-school stimulant vs modern GLP-1
Phentermine vs Semaglutide
Updated September 28, 2026
The short answer
Semaglutide produces much larger and longer-lasting weight loss and is approved for long-term use. Phentermine is a cheap stimulant approved only for short-term use, with average losses of around 5% over 12 weeks. Phentermine can make sense as a short, low-cost kick-start for some people, but it is not a long-term plan and is not suitable for anyone with heart disease.
Phentermine vs Semaglutide at a glance
| Phentermine | Semaglutide | |
|---|---|---|
| Drug type | Stimulant appetite suppressant | GLP-1 receptor agonist |
| Approved since | 1959 | 2017 (diabetes), 2021 (weight) |
| Average weight loss | About 5% over 12 weeks | 14.9% over 68 weeks |
| How long you can take it | Short term, usually up to 12 weeks | Long term |
| How you take it | Capsule each morning | Weekly injection or daily tablet |
| Heart effects | Raises heart rate and blood pressure | Cut major cardiovascular events by 20% in the SELECT trial |
| Controlled substance | Yes, Schedule IV | No |
| Cost | Very low | Higher |
Two very different ways to reduce appetite
Phentermine is chemically related to amphetamine. It stimulates the nervous system, which suppresses appetite and raises energy, but also heart rate and blood pressure. Because it can be misused, it is a controlled substance.
Semaglutide copies a natural gut hormone. It slows how quickly your stomach empties and acts on appetite centres in the brain, so you feel full sooner and think about food less. It is not a stimulant.
Results and how long they last
Phentermine works fast, with appetite changes within days, but its effect fades as the body adapts, which is one reason it is approved only for a few weeks. Much of the weight tends to return once it stops unless habits have changed.
Semaglutide takes longer to build up because the dose is raised gradually, but the loss keeps coming for most of the first year. Weight also tends to return after stopping, which is why it is usually treated as a long-term medicine.
Safety differences that matter
Phentermine is not suitable for people with heart disease, uncontrolled high blood pressure, an overactive thyroid, glaucoma or a history of substance misuse. Insomnia, dry mouth and a racing heart are common.
Semaglutide's main side effects are digestive. In the SELECT trial of people with heart disease and overweight, it reduced heart attacks, strokes and cardiovascular deaths by 20%. It is not suitable for people with a history of medullary thyroid cancer or MEN 2.
Common questions
This comparison is for general information and is not medical advice. Talk to a licensed clinician before starting, stopping or switching any medication.
Not sure which is right for you?
A licensed clinician reviews your history and labs and recommends the option that fits, if any.