Wegovy (semaglutide) and phentermine are both FDA-approved specifically for weight loss, but they represent two very different eras and mechanisms of obesity treatment: Wegovy is a modern GLP-1 receptor agonist for sustained long-term use, while phentermine is a decades-old stimulant-based appetite suppressant intended for short-term use. This guide compares them directly.
This is general information, not medical advice.
Wegovy vs Phentermine at a glance
| Wegovy (semaglutide) | Phentermine | |
|---|---|---|
| Drug class | GLP-1 receptor agonist | Sympathomimetic amine |
| FDA-approved for | Chronic weight management, cardiovascular risk reduction, pediatric obesity (12+), MASH | Short-term (a few weeks) weight loss adjunct |
| Controlled substance | No | Yes, Schedule IV |
| Administration | Once-weekly injection, or a once-daily oral tablet (newer formulation) | Once-daily oral tablet |
| Intended duration | Long-term, ongoing | Short-term |
Both are actually approved for weight loss — but very differently
This is the one comparison in this cluster where both drugs carry a genuine FDA weight-loss indication, but the scope differs enormously. Wegovy is indicated to reduce excess body weight and maintain weight reduction long-term in adults with obesity, or overweight with at least one weight-related condition, and its label has expanded over time to include reducing risk of major cardiovascular events, treatment in adolescents 12 and older, and even an accelerated approval for MASH (a liver condition), reflecting a broad and still-growing label. As of 2025, an oral Wegovy tablet became available as well, alongside the original injection, giving patients a needle-free option for the first time.
Phentermine’s approval, by contrast, has stayed essentially the same since it was introduced: short-term, a few weeks, as an adjunct to diet, exercise, and behavior change, for patients meeting specific BMI thresholds.
How they work
Semaglutide (Wegovy) mimics the GLP-1 hormone, which increases insulin secretion, decreases glucagon, slows gastric emptying, and acts on appetite-regulating centers in the brain, collectively reducing hunger and food intake over weeks of dose escalation.
Phentermine works as a central nervous system stimulant, suppressing appetite through a sympathomimetic mechanism related to amphetamines, producing a faster but generally less dramatic weight-loss effect than what’s been documented in Wegovy’s large clinical trials.
Effectiveness: what the trials showed
Wegovy’s weight-loss trials, including several 68-week randomized, placebo-controlled studies, showed average weight loss meaningfully larger than what’s typically documented for older appetite suppressants like phentermine in comparable trial durations, a major reason for GLP-1 drugs’ current prominence in obesity treatment. Phentermine’s own label notes that the amount of weight loss with any “anorectic” drug varies by trial and depends on variables beyond the drug itself, including the treating physician, population, and diet prescribed, an honest, decades-old caveat that still applies.
Side effects
Wegovy commonly causes nausea, vomiting, diarrhea, and constipation, and carries the same boxed thyroid C-cell tumor warning (based on rodent data, human relevance undetermined) shared across the semaglutide product line.
Phentermine commonly causes insomnia, elevated heart rate and blood pressure, dry mouth, and nervousness, plus carries dependence potential as a Schedule IV controlled substance.
Who each might suit
Factors that may point toward Wegovy:
- A need for substantial, sustained long-term weight loss
- Co-occurring cardiovascular risk factors, given its cardiovascular risk-reduction indication
- Adolescent obesity treatment (ages 12+), where phentermine is not the typical approach
- A preference for a needle-free option, now available via the oral tablet
Factors that may point toward phentermine:
- A shorter-term, lower-cost weight-loss need
- No tolerance for GI side effects, where a stimulant-based mechanism might be preferred instead
- Prior good response specifically to stimulant-type appetite suppression
Frequently asked questions
Is there an oral (pill) version of Wegovy now?
Yes, as of 2025 an oral Wegovy tablet is available alongside the original once-weekly injection, giving patients a needle-free semaglutide option specifically for weight loss.
Which produces more weight loss, Wegovy or phentermine?
Wegovy’s large clinical trials have generally shown greater average weight loss than what’s typically documented with phentermine, though individual results vary and trial designs differ.
Is phentermine addictive?
It’s a Schedule IV controlled substance with some dependence potential, related to its stimulant mechanism, though generally considered lower-risk than Schedule II stimulants.
Can teenagers use either drug for weight loss?
Wegovy is approved for adolescents 12 and older with obesity; phentermine’s approval doesn’t specifically extend to this age group in the same documented way.
Does Wegovy treat anything besides obesity?
Yes, its label has expanded to include reducing cardiovascular risk in adults with obesity/overweight and established cardiovascular disease, and an accelerated approval for MASH, a liver condition.