Phentermine vs Ozempic and Wegovy: Is Phentermine a GLP‑1?

No. Phentermine is a sympathomimetic amine with activity similar to amphetamine; it does not act on the GLP-1 receptor.1 Its label groups it with the drugs commonly called "anorectics."1 GLP-1 drugs such as Wegovy instead activate the receptor for GLP-1, a hormone that regulates appetite and calorie intake.2 For how GLP-1 drugs work, see what is GLP-1.

Phentermine's label describes central nervous system stimulation and raised blood pressure as typical of its drug class.1 Its pharmacologic activity is similar to amphetamine, and it carries a warning about abuse and dependence.1

Yes. Phentermine is a Schedule IV controlled substance.1 Its label asks prescribers to give the smallest amount feasible at one time to limit the risk of overdose.1 Ozempic, Wegovy and Zepbound are prescription drugs, and their labels assign no controlled-substance schedule.2,3,4

Phentermine acts on the nervous system like other amphetamine-class drugs, while semaglutide activates GLP-1 receptors in brain areas that regulate appetite.1,2 The Wegovy label explains that GLP-1 is a natural regulator of appetite and calorie intake, and that semaglutide reaches neurons involved in food intake in animal studies.2

Phentermine's own label says it has not been established that appetite suppression is its main action; other nervous system or metabolic effects may also be involved.1 The label does not describe phentermine as a fat-burning drug.1 It notes that tolerance to the appetite-suppressing effect usually develops within a few weeks, and that phentermine should then be stopped rather than taken at a higher dose.1

Phentermine is approved as a short-term add-on of a few weeks, while Wegovy is approved to reduce weight and maintain the loss long term.1,2 This is the biggest practical difference between the two.

The phentermine label explains why: the studies behind it lasted only a few weeks, while obesity develops over years.1 Wegovy's main trials ran 68 weeks, and one tested what happened when people stopped: those switched to placebo after 20 weeks regained 6.9% of their weight by week 68, while those who stayed on Wegovy lost a further 7.9%.2

In its label, phentermine's extra weight loss over placebo is described as only a fraction of a pound a week, while Wegovy's label reports 14.9% average weight loss at 68 weeks against 2.4% on placebo.1,2 No head-to-head trial of the two appears in either label.

Phentermine's label adds three cautions about its data:1

• Weight loss was fastest in the first weeks for both drug and placebo groups, then slowed.
• Results varied between trials and depended partly on the doctor, the patients and the diet.
• Trials lasted only weeks, so the long-term effect over diet alone is uncertain.

On Wegovy 2.4 mg, 83.5% of adults lost at least 5% of their weight and 66.1% lost at least 10% in the main trial.2 The Wegovy tablet produced 13.6% average loss at 64 weeks.2 Zepbound, a tirzepatide drug, produced 20.9% at its 15 mg dose over 72 weeks.4 See Wegovy vs Zepbound for that comparison.

The label does not give a typical amount; it says the added loss over placebo is a fraction of a pound a week, and that loss slows after the first weeks.1

Phentermine's side effects are mainly stimulant effects on the heart and nervous system, while semaglutide's are mainly digestive.1,2

Phentermine's label lists palpitations, fast heart rate, raised blood pressure, restlessness, dizziness, insomnia, tremor, headache, dry mouth, unpleasant taste, diarrhea and constipation.1 Rare cases of primary pulmonary hypertension and heart valve disease have been reported.1 It should not be used by people with heart disease, uncontrolled high blood pressure, hyperthyroidism, glaucoma, agitated states or a history of drug abuse, or during pregnancy or breastfeeding.1

The most common Wegovy side effects are nausea, diarrhea, vomiting, constipation and abdominal pain.2 Its serious warnings include a boxed thyroid C-cell tumor warning, pancreatitis and gallbladder disease.2 See Wegovy side effects and Ozempic side effects for the full tables.

The phentermine label says it should not be combined with other weight-loss drugs, because the safety and effectiveness of such combinations have not been established.1 It also warns that people on insulin or diabetes pills may need lower doses of those drugs.1 Ask your prescriber before combining any weight-loss medicines.

What is phentermine used for? As a short-term add-on to diet, exercise and behavior change for weight loss in people with a BMI of 30 or more, or 27 or more with conditions such as controlled high blood pressure, diabetes or high cholesterol.1

Is phentermine like Adderall? Phentermine's label says its activity is similar to amphetamine, and it warns about abuse and dependence.1 It is approved only for weight loss.1

Can I take more than 37.5 mg of phentermine a day? The label sets the usual adult dose at 37.5 mg a day and warns that the recommended dose should not be exceeded.1 Some patients take 18.75 mg once or twice a day.1

When should phentermine be taken? Before breakfast or 1 to 2 hours after; late-evening doses are avoided because they can cause insomnia.1

Is Ozempic stronger than phentermine for weight loss? Ozempic is not approved for weight loss; Wegovy is the semaglutide brand that is.3,2 Wegovy's label reports far larger and longer-lasting average loss than phentermine's label describes.2,1 See Ozempic vs Wegovy.

If you are weighing a long-term GLP-1 against a short course of phentermine, start with who qualifies for a GLP-1, then compare prices in our Wegovy cost guide and our cheapest ways to get Wegovy. For oral options, see our GLP-1 pill rankings and the explainer on the Wegovy pill.

This article is for information only and is not medical advice. Phentermine is a controlled substance and GLP-1 medications are prescription drugs with real risks. Talk to a licensed clinician before starting, stopping or combining any weight-loss medication.