What’s the difference between Wegovy and phentermine?

Weight-loss medications — especially GLP-1s — are getting a lot of attention right now, but FDA-approved drugs to help people manage their weight have been around for decades. Wondering how the newer drugs stack up against the older ones? Let’s take Wegovy vs. phentermine, for example. Here’s how these two medications really compare.

What is Wegovy?

Wegovy is a brand name for the GLP-1 medication semaglutide that has been FDA-approved for weight loss. Given as a weekly injection, Wegovy works by mimicking GLP-1, short for glucagon-like peptide 1, a hormone that your body naturally produces whenever you eat. This hormone signals the pancreas to release insulin as a way to control blood sugar, prompts feelings of fullness, and reduces your appetite.

What is phentermine?

Phentermine (sold under the brand names Adipex-P, Lomaira, and Ionamin) is an amphetamine-like medication (a.k.a. a stimulant) taken in pill form. It was approved in 1959, making it the oldest FDA-approved weight-loss medication still on the market, and has traditionally been the most commonly prescribed anti-obesity drug in the United States, according to the Obesity Medicine Association. While the idea of taking anything resembling an amphetamine can seem scary, phentermine has a weaker effect than true amphetamines like the ones used to treat ADHD. As a result, they are less likely to lead to dependence or have withdrawal symptoms after you use them.

"Phentermine works directly in the appetite center of the brain to reduce hunger," says Dr. Robert Kushner, M.D., professor of medicine and medical education at Northwestern University Feinberg School of Medicine in Chicago. It does this by stimulating neurons in the central nervous system in a way that releases two neurotransmitters related to appetite: dopamine and norepinephrine. As levels of these increase, so does leptin regulation — a hormone that tells your brain you’re full.

Key differences between Wegovy and phentermine

While both Wegovy and phentermine are used for weight management, these medications have key differences, says Dr. Paunel Vukasinov, M.D., an internal medicine doctor at Medical Offices of Manhattan in New York. Most notably, the way they help you lose weight is completely different, with Wegovy being a GLP-1 agonist that reduces hunger, manages blood sugar, and slows gastric emptying while phentermine only targets hunger. And that’s not all. "Wegovy is a once-weekly injection that was developed for long-term weight management, while phentermine is a daily oral medication only meant for short-term use of up to three months, although it may be continued for longer," Vukasinov says. The majority of people who take phentermine stay on it for just three to six weeks. This is because phentermine has some unwanted side effects that can include feeling jittery or having difficulty sleeping and using it may be associated with increased heart rate and blood pressure. Phentermine is often thought of as the kickstart to weight loss and the first sprint, while Wegovy is more like a marathon, with a slow and steady pace.

Wegovy vs. phentermine: uses

Both phentermine and Wegovy are used for weight loss, but Wegovy may have more overall benefits. Research shows that GLP-1 medications such as semaglutide show promise for better kidney function, reduced inflammation throughout the body, and a healthier heart, says Kushner. For instance, one study found that weekly treatment with semaglutide reduced risk of a combination of stroke, heart attack, and death from cardiovascular causes by 20% compared to a placebo. Those advantages are due to the weight loss in part, but there are other mechanisms as well. For example, semaglutide can lead to better blood flow, which can help organs like the heart and kidneys. 

Phentermine is taken for such a short time that its benefits are mostly weight loss, Vukasinov says.

Wegovy vs. phentermine: dosages and administration

Wegovy is given as an injection once a week, and you'll likely start on the lowest dosage of 0.25mg. Unless you're experiencing significant side effects, your provider will probably increase the dose every four weeks until you're at the dose that works best for you — 1.7mg or 2.4mg for most people.

By contrast, phentermine is taken daily as an oral medication, usually in a dosage of 37.5mg, but there is a lower dosage of 15mg for those with health concerns such as renal impairment, which is when your kidney doesn’t function correctly. You tend to start on one dose and stay on that dose, instead of gradually increasing it like you do with Wegovy. The medication can be taken with or without food but should not be taken in the evening, since it can raise your risk of insomnia.

Efficacy of Wegovy vs. phentermine

Wegovy can lead to greater weight loss than phentermine. Research in The New England Journal of Medicine found that people taking semaglutide (the active ingredient in Wegovy) lost an average of 15% of their body weight in a little over a year. And as long as you continue to take the medication, maintaining that weight loss is likely. Research on phentermine suggests 5% is the upper limit for weight loss, and since it’s a short-term medication, it does not help you maintain weight loss over the long term. 

“When a patient may have less weight to lose in general, or they are reluctant to be on longer-term medication, phentermine can be a good option,” says Vukasinov. “Also, someone may start with phentermine as a way to ‘kick off’ a weight-loss effort, then possibly transition to an option like Wegovy afterward.”

Wegovy vs. phentermine: cost

There is a pretty big difference in terms of how much you’ll pay to get Wegovy vs. phentermine. Phentermine can be sold as a generic drug and usually costs between $10 to $30 for a month’s supply, says Kushner. Semaglutide, on the other hand, is much more expensive due to there being no generic options; it typically costs over $1000 for a month’s supply without insurance. Insurance can significantly reduce the out-of-pocket cost if Wegovy is covered, but at the moment, many plans do not cover anti-obesity medications.

Wegovy vs. phentermine: side effects

Similar to any medication — even over-the-counter options like aspirin — Wegovy may cause side effects, which can range from mild and uncomfortable to more serious.

The most common side effects of GLP-1 medications like Wegovy are nausea, vomiting, diarrhea, and acid reflux (find the full list of side effects here). More serious but less common side effects include pancreatitis, gallbladder disease, and worsening of diabetic eye disease. Those with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasias should not use them. 

The side effects of phentermine include dry mouth, unpleasant taste, diarrhea, constipation, and vomiting (find the full list of side effects here). More serious side effects that should prompt medical attention are increased blood pressure, heart palpitation, chest pain, shortness of breath, and swelling of the legs and ankles. Unlike Wegovy, phentermine also comes with risk of addiction. This is why it’s only prescribed on a short-term basis.

Is Wegovy better than phentermine?

There isn’t really an answer to that question. While Wegovy can lead to more weight loss than phentermine, the decision of which medication to use is based on multiple factors that include effectiveness, safety, side effects, affordability, and availability, says Kushner. "The healthcare professional who is responsible for providing the prescription should take all of these factors into consideration when presenting medication options," he adds

The bottom line

Both phentermine and Wegovy can be useful weight-loss medications, but it's helpful to keep in mind that they work in different ways and are taken for different time periods. While phentermine is widely utilized for short-term appetite suppression, Wegovy will very likely lead to more weight loss but is considered a long-term commitment for weight maintenance. "Phentermine is a more cost-effective alternative and an option for people who would benefit from modest weight loss, do not have coverage for semaglutide, or do not want to take an injectable medication," says Kushner.