Phentermine vs Ozempic: phentermine is a stimulant pill approved only for short-term weight loss (a few weeks), while semaglutide, the drug in Ozempic, is a weekly GLP-1 injection used long term.[1][2] At the Wegovy dose, semaglutide produced 14.9% average weight loss over 68 weeks; a 12-week phentermine trial found 8.1 kg versus 1.7 kg on placebo.[4][5] Phentermine is a Schedule IV controlled substance; semaglutide is not.[1] Combining them has not been shown to be safe or effective.[1]
Ozempic itself is approved for type 2 diabetes; semaglutide for weight loss is sold as Wegovy.[2][3] See our semaglutide profile for background.
Phentermine vs Ozempic at a glance
| Phentermine | Ozempic / Wegovy (semaglutide) | |
|---|---|---|
| Type of drug | Sympathomimetic (amphetamine-related) appetite suppressant[1] | GLP-1 receptor agonist[2] |
| Form | Daily tablet or capsule[1] | Weekly injection; daily pill (Wegovy)[3] |
| Approved use | Short-term weight loss (a few weeks)[1] | Ozempic: type 2 diabetes; Wegovy: long-term weight management[2][3] |
| Controlled substance | Yes, Schedule IV[1] | No |
| Weight loss evidence | −8.1 kg vs −1.7 kg at 12 weeks (one trial)[5] | −14.9% vs −2.4% at 68 weeks (STEP 1)[4] |
| Heart effects | Raises blood pressure and heart rate; not for heart disease[1] | Lowered heart events in SELECT[10] |
| Tolerance | Appetite effect usually fades within weeks[1] | Weight loss sustained over 68 weeks in STEP 1[4] |
| Generic | Yes | No US generic |
How they work
Phentermine is related to amphetamine. It stimulates the central nervous system and reduces appetite, though the label notes its exact weight-loss mechanism is not fully established. Tolerance to its appetite effect usually develops within a few weeks, and the label says to stop it if that happens rather than raise the dose.[1]
Semaglutide copies GLP-1, a gut hormone. It reduces appetite, slows stomach emptying and, in diabetes, increases insulin when blood sugar is high.[2] In STEP 1, people taking it for 68 weeks kept losing weight well beyond the first few months.[4]
Weight loss compared
| Study | Drug | Length | Result |
|---|---|---|---|
| Korean randomized trial | Phentermine 30 mg (controlled release) | 12 weeks | −8.1 kg vs −1.7 kg placebo; 62.5% lost 10% or more[5] |
| US health-record cohort | Phentermine used more than 12 months | 24 months | 7.4% more weight loss than people who used it 3 months or less[6] |
| STEP 1 | Semaglutide 2.4 mg weekly | 68 weeks | −14.9% vs −2.4% placebo[4] |
| SUSTAIN FORTE (diabetes) | Ozempic 2 mg weekly | 40 weeks | −6.4 kg[12] |
Phentermine has no large, long-term randomized trial of its own, and its label limits use to a few weeks. In April 2026, the American College of Physicians recommended semaglutide and tirzepatide as first-line obesity drugs; phentermine alone was not among its recommended options, though the combination pill phentermine-topiramate (Qsymia) was ranked second.[9]
Side effects and safety
Phentermine, from its label:[1]
- Raised blood pressure, fast heartbeat and palpitations
- Restlessness, insomnia, dizziness, dry mouth, headache
- Rare but serious: primary pulmonary hypertension and heart valve disease
- Not for people with heart disease, stroke, uncontrolled high blood pressure, overactive thyroid, glaucoma, agitation, a history of drug misuse, pregnancy or breastfeeding
- Should not be taken in the late evening, because of insomnia
Semaglutide, from its labels:[2][3]
- Nausea, vomiting, diarrhea and constipation, mostly while the dose is rising
- Warnings for pancreatitis, gallbladder disease, kidney injury from dehydration and low blood sugar with insulin or sulfonylureas
- Boxed warning about thyroid C-cell tumors in rodents; not for people with a personal or family history of medullary thyroid cancer or MEN 2
Controlled-substance rules
Phentermine is a Schedule IV controlled substance. The label warns that it has potential for abuse and dependence, and that stopping after prolonged high doses can cause extreme fatigue and depression.[1] In practice, controlled-substance status means tighter prescription and refill rules. Semaglutide is not a controlled substance.
Can you take phentermine and Ozempic together?
The evidence is thin:
- Phentermine label: combining phentermine with any other weight-loss drug, prescription or over the counter, is not recommended because safety and effectiveness have not been established.[1]
- Wegovy label: advises against combining it with other GLP-1 drugs, but does not specifically address phentermine.[3]
- The only randomized trial we found added phentermine 15 mg or placebo to liraglutide 3 mg (an older GLP-1) in 45 adults who had already lost 12.6% on liraglutide. After 12 weeks, the phentermine group lost 1.6% more versus 0.1% on placebo, a difference that was not statistically significant. The authors said the combination was well tolerated but added no clinically meaningful weight loss.[7]
- No randomized trial of phentermine with semaglutide has been published.
Using them together would be off-label. If you are considering it, a prescriber needs to weigh heart risk, blood pressure, other medicines and the lack of evidence, and to plan how progress and side effects will be checked.
Phentermine-topiramate (Qsymia): the long-term phentermine option
Qsymia combines low-dose phentermine with topiramate and is approved for long-term weight management in adults and teens 12 and older.[8] In the CONQUER trial, the top dose produced 9.8% weight loss at 56 weeks versus 1.2% on placebo.[11] It requires monthly pregnancy tests for people who can become pregnant.[8] See GLP-1 alternatives for how it compares with other options.
Which might suit you?
The choice depends on your health and goals, and a prescriber should make it with you:
- Long-term weight management: semaglutide (as Wegovy) is approved for chronic use and has the stronger evidence. Phentermine alone is labeled only for a few weeks.[1][3]
- Heart disease or high blood pressure: phentermine is not for people with heart disease or uncontrolled high blood pressure, while Wegovy lowered heart events in people with heart disease.[1][10]
- Type 2 diabetes: Ozempic is approved for blood sugar control; phentermine may change insulin or diabetes-pill needs.[2][1]
- History of substance misuse: phentermine is contraindicated.[1]
- Cost is the main barrier: generic phentermine is usually cheaper, but its benefit is short-lived as tolerance develops.[1]
Other older options are compared in Contrave vs Ozempic.
Cost
Phentermine is an older drug available as generics. Semaglutide has no US generic; in October 2026, NovoCare’s self-pay price was $349 a month for Wegovy (most doses) and for Ozempic 0.25 to 1 mg.[13] See Wegovy vs Ozempic for coverage differences and semaglutide cost for savings programs.
Prices checked October 2026. They change often; confirm with your pharmacy or plan.
