Wegovy produces significantly more weight loss than phentermine and can be used long-term, but it costs far more and comes with a different set of side effects. In a large meta-analysis of 143 randomized trials, semaglutide (the active ingredient in Wegovy) led to an average body weight reduction of about 11.4%, while phentermine combined with topiramate produced roughly 8%. Phentermine on its own typically delivers more modest results, in the range of 5% to 7% over a few months. The right choice depends on your health history, how much weight you need to lose, and what you can afford.
How They Work Differently
Wegovy and phentermine attack appetite through completely different pathways. Wegovy is a once-weekly injection that mimics a gut hormone called GLP-1. This hormone signals your brain to feel full after eating, slows down how fast food leaves your stomach, and reduces cravings. The effect builds gradually over several weeks as the dose is increased.
Phentermine is a daily oral pill that works as a stimulant. It triggers the release of brain chemicals that suppress hunger, similar to how amphetamines work (though it’s much milder). You feel less hungry, but the mechanism also raises your heart rate and blood pressure slightly, which is one reason it’s only approved for short-term use.
Weight Loss Results
Wegovy’s clinical trials ran for 68 weeks, and participants lost an average of about 15% of their starting body weight when combined with diet and exercise changes. That translates to roughly 35 pounds for someone who starts at 230 pounds. The weight loss is gradual, with most of it happening in the first 9 to 12 months before plateauing.
Phentermine’s FDA label is explicit: it’s approved for only “a few weeks” of use. Most clinical data on phentermine covers 12-week windows, where patients typically lose 5% to 7% of body weight. Some doctors prescribe it for longer periods off-label, but the FDA itself notes that “the total impact of drug-induced weight loss over that of diet alone must be considered clinically limited” given the short study durations. The practical reality is that many people regain weight after stopping phentermine, because obesity doesn’t resolve in a few weeks.
The meta-analysis comparing weight loss medications head-to-head ranked semaglutide first among all options, with an 11.4% average reduction in body weight beyond what lifestyle changes alone achieved. Phentermine-topiramate (a combination pill sold as Qsymia) came in second at about 8%. Phentermine alone ranked lower.
Side Effects to Expect
The two drugs cause very different types of side effects. Wegovy’s most common problems are gastrointestinal: nausea, vomiting, diarrhea, and constipation. These tend to be worst during the dose-escalation period (the first 4 to 5 months) and often improve over time. Wegovy also carries the FDA’s most serious warning, a boxed warning, for a potential link to a rare type of thyroid cancer called medullary thyroid carcinoma. If you or a close family member has a history of this cancer, or a genetic condition called MEN2, Wegovy is off the table.
Phentermine’s side effects lean toward stimulant-related issues: increased heart rate, elevated blood pressure, insomnia, dry mouth, restlessness, and jitteriness. Because it’s chemically related to amphetamines, it also carries a risk of dependence, which is another reason for the short-term use restriction. People with a history of substance misuse, glaucoma, an overactive thyroid, or significant heart problems should not take phentermine.
Interestingly, both medications carry similar rates of people quitting due to side effects. In the meta-analysis, semaglutide had a discontinuation odds ratio of 2.45 and phentermine-topiramate came in at 2.40, meaning people on either drug were roughly 2.4 times more likely to stop treatment because of side effects compared to placebo.
How Long You Can Take Them
This is one of the biggest practical differences. Wegovy is approved for ongoing, chronic use. Obesity is a long-term condition, and Wegovy is designed to be taken indefinitely to maintain weight loss. When people stop Wegovy, most regain a significant portion of the weight they lost within a year.
Phentermine’s FDA approval limits it to a few weeks. In practice, many prescribers use it for 3 months at a time, sometimes with breaks in between. But there’s no long-term safety data supporting continuous use, and its stimulant properties make indefinite prescribing risky. This creates a fundamental problem: phentermine may help you lose weight in the short term, but it doesn’t offer a plan for keeping it off.
Cost Comparison
Phentermine wins decisively on price. Because generic versions are widely available, you can fill a monthly prescription for as little as $13. Even without insurance, phentermine rarely exceeds $30 to $50 per month.
Wegovy is dramatically more expensive. Without insurance, GoodRx lists introductory pricing around $199 per month for the injection, rising to $349 per month for ongoing fills. If you have commercial insurance that covers Wegovy and you meet eligibility requirements, a manufacturer savings card can bring the cost down to as little as $25 per month. But coverage varies widely. Many insurance plans still exclude weight loss medications, and Medicare Part D only recently began covering Wegovy for certain indications. Wegovy has no generic version.
Over a year, the difference is stark: phentermine might cost $150 total, while Wegovy could run anywhere from $300 (with ideal insurance) to over $4,000 out of pocket.
Who Each Medication Suits Best
Both medications are approved for adults with a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related health condition like high blood pressure, type 2 diabetes, or high cholesterol. But the populations they serve best differ in practice.
Wegovy is generally the stronger option for people who need substantial, sustained weight loss. It’s particularly relevant for people with type 2 diabetes or cardiovascular risk factors, since semaglutide has shown heart-protective benefits in clinical trials. It’s also a better fit if you’ve tried short-term approaches before and regained the weight.
Phentermine may work well as a short-term jumpstart, especially if cost is a major barrier or if you only need to lose a modest amount of weight. It’s also sometimes used as a bridge while waiting for insurance approval on other medications. However, it’s not appropriate if you have heart disease, uncontrolled high blood pressure, glaucoma, hyperthyroidism, or a history of drug misuse.
Wegovy should be avoided if you have a personal or family history of medullary thyroid carcinoma, MEN2 syndrome, or a history of pancreatitis. People with diabetic retinopathy need careful monitoring on Wegovy, as rapid blood sugar changes can temporarily worsen eye problems.
The Bottom Line on Effectiveness
By nearly every clinical measure, Wegovy produces more weight loss and offers a long-term treatment framework that phentermine can’t match. But “better” depends on your situation. If you have insurance coverage and need to lose a significant amount of weight, Wegovy is the more effective tool. If you’re paying out of pocket, need a short-term boost, or can’t take Wegovy due to medical history, phentermine remains a reasonable and far cheaper option. Neither medication works well without changes to diet and physical activity, and both cause weight regain after stopping.