Wegovy vs. Phentermine for Weight Loss: 6 Key Differences to Remember

Many prescription medications are available to help you lose weight. One of them is phentermine (Adipex-P), an oral medication that’s been around for decades as a short-term option for weight loss. But recently, injectable medications — such as Wegovy (semaglutide) — have hit the market, too. And they’ve garnered quite a bit of attention.

These newer treatments not only help people reach their body weight goals, but they can also help lower the risk of heart attack, stroke, and heart-related death. This has caused many people to question if these new medications might be a better choice.

Wegovy and phentermine can both be effective treatments. But they manage body weight in different ways. And there are differences when it comes to dosing instructions, results, and side effects, too. 

Here, we’ll review six key differences between Wegovy and phentermine.

1. Wegovy and phentermine promote weight loss in separate ways

Wegovy and phentermine can both affect your appetite, leading to weight loss from eating fewer calories. But they do this in different ways.

Wegovy is a glucagon-like peptide-1 (GLP-1) agonist. GLP-1 agonists mimic an incretin hormone called GLP-1 that your body makes naturally. This hormone plays a role in appetite and digestion regulation, causing you to feel more full and eat fewer calories — and eventually lose weight.

Wegovy does this by slowing down how quickly food leaves your stomach (gastric emptying). This can help you feel fuller faster and stay full for longer. Wegovy is also thought to help you feel less hungry and have less cravings.

Phentermine works differently. It promotes the release of certain chemicals in your brain, such as norepinephrine and dopamine, that reduce feelings of hunger. People who take phentermine tend to eat less, which can result in weight loss.

Keep in mind: Wegovy and phentermine work best when combined with a healthy diet and exercise.

2. Wegovy and phentermine have unique dosages and are administered differently

Wegovy is an injectable product and phentermine is a pill. Each one has specific instructions on taking your dose.

Wegovy is a once-weekly injectable medication that comes in a single-use, prefilled pen. The pens are available in five dosages. The dose is preset for each device and the needle comes attached to the pen.

Wegovy is safe to administer on your own at home. You can inject your dose just under the skin (subcutaneously) into your stomach or the top of your thigh. You can also receive it in your upper arm, but due to the awkward angle, it’s best to have someone else inject it for you here. You should avoid injecting into the same spot every time. Instead, change your injection site with each dose to help avoid injection site irritation.

The typical starting dose of Wegovy is 0.25 mg once weekly for 4 weeks (1 month). If you’re tolerating the medication well, your dose will slowly increase every 4 weeks until you reach a maintenance dose of 2.4 mg once weekly.

Phentermine is an oral product that comes as a tablet and capsule. It’s also available as an orally disintegrating tablet (ODT) that dissolves on the tongue. Phentermine tablets are available in a 37.5 mg formulation, while the capsules are available in 15 mg, 30 mg, and 37.5 mg formulations.

Most forms of phentermine are taken once daily before breakfast or 1 to 2 hours after breakfast. It’s also available as a low-dose 8 mg tablet called Lomaira, taken 3 times daily 30 minutes before meals.

Keep in mind: Phentermine is FDA approved for people ages 17 and older, while Wegovy is approved for adults and children ages 12 and older. But both medications are only approved for people who meet certain body mass index (BMI) criteria.

3. Wegovy is approved for chronic weight management and cardiovascular risk reduction, while phentermine is not

Wegovy is a chronic weight management medication. Long-term studies have found that people who use it are able to maintain their weight loss with continued treatment. However, regaining body weight is possible after stopping it.

On the other hand, phentermine is only meant for short-term use. It’s a controlled substance that’s approved for up to 12 weeks (3 months) of use at a time. This is because your body can develop a tolerance to phentermine after a few weeks, so its effects can weaken. Also, the longer you use phentermine, the risk of misuse and dependency increases.

That being said, some people still take phentermine for longer than 3 months at a time. But this should only be done under the close supervision of a trained healthcare professional.

Phentermine is also available in a combination pill with topiramate called Qsymia. Unlike phentermine on its own, Qsymia is approved for chronic weight management. It has been shown to be safe and effective in longer-term studies.

Cardiovascular risk reduction

Wegovy gained another FDA-approved use in March 2024. It’s now approved to lower the risk of serious heart-related events in adults who have heart disease and are considered either overweight or obese. Wegovy helps lower the risk of heart attacks, strokes, and heart-related death in this group. It’s the first weight-loss medication to be officially approved for this use.

Phentermine isn’t approved or recommended for this purpose.

4. Wegovy may result in more weight loss than phentermine

Wegovy and phentermine haven’t been directly compared for weight loss. But data from individual studies suggest that people tend to lose more body weight with Wegovy.

Research suggests that phentermine can help people lose 3% to 5% of their initial body weight (or more, in some cases).

By comparison, adults receiving Wegovy in clinical trials experienced an average weight reduction of almost 15% of their initial body weight. These results were evaluated over a period of 68 weeks (16 months). Adolescents saw a similar reduction in body weight over the same period.

5. There are different risks and side effects between Wegovy and phentermine

Wegovy and phentermine have different side effect profiles. 

Stomach-related side effects tend to happen most frequently with Wegovy. Examples include:

• Nausea and vomiting

• Stomach pain

• Constipation

• Diarrhea

• Gas and bloating

• Burping

Wegovy can also cause headaches or fatigue. The good news is that most of these side effects tend to go away as your body gets used to the medication.

While rare, Wegovy has been linked to a few other serious risks and side effects more common with long-term use. These include:

• Inflammation of the pancreas (pancreatitis)

• Worsening of diabetes-related eye problems

• Gallbladder problems

• Kidney problems

• Faster heart rate

Phentermine has less stomach-related concerns. The most common phentermine side effects include dry mouth and trouble sleeping. It can also cause headaches and shakiness. These side effects may go away as your body gets used to the medication.

Wegovy’s boxed warning

Wegovy carries a boxed warning about thyroid C-cell tumors occurring in animal studies. This is the FDA’s most serious type of medication warning. Although this specific cancer risk hasn’t been confirmed in people, you’ll want to avoid taking Wegovy if you have a personal or family history of certain types of thyroid cancer.

By comparison, phentermine doesn’t have any boxed warnings. But there have been rare, life-threatening cases of dangerously high blood pressure, leaking heart valves, and high blood pressure in the lungs.

6. Phentermine is available as a lower-cost generic, but Wegovy isn’t

When it comes to weight-loss medications, cost is a common concern. One of the best ways to save on your prescriptions is to fill a generic version of the medication.

Generic versions of phentermine are available. But Wegovy is only available as a brand-name medication. The best way to save on Wegovy is to use insurance — if you have it and if the medication’s covered.

If Wegovy’s cost is still high, you could also save with a copay savings card. If you have commercial insurance and meet eligibility requirements, Wegovy’s price is as little as $0 for a 28-day supply when you use the manufacturer’s savings card.

How to choose the best weight-loss medication

One weight-loss medication isn’t widely considered better than the rest. The best weight-loss medication for you will likely come down to a few key factors, including:

• Age

• Treatment goals

• Personal preferences

• Underlying health conditions

• Side effects

• Cost

You and your healthcare team can work together to decide which weight-loss medication is best for you. It’s not recommended to take Wegovy and phentermine together.

Why trust our experts?

Written by:

Nikki Javit, PharmD

Nikki Javit, PharmD, is a clinical pharmacist with certifications in international travel medicine, immunization delivery, and pharmacogenomics. She received her bachelor’s of science in biology from Indiana University Bloomington and her doctor of pharmacy degree from Butler University.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Alyssa Billingsley, PharmD

Alyssa Billingsley, PharmD, is the director of pharmacy content for GoodRx. She has over a decade of experience as a pharmacist and has worked in clinical, academic, and administrative roles.

Our editorial standards

Meet our experts