Ozempic vs. phentermine: which is better for weight loss?

Losing weight is no easy feat, and the reasons for that are complex, having little to do with how much willpower you have (despite popular belief).

We understand that obesity is a disease with many contributing factors, so it makes sense that it often takes multiple approaches to treat it. Adjusting eating patterns and getting more activity is important no matter what, but many people also need medication to help get their weight under control. 

In this article, we’ll look at two drugs often prescribed for weight loss—Ozempic (off-label) and phentermine (approved for weight loss)—to see how they compare.

Ozempic (you may have heard it called by its generic name, semaglutide) is a medication that’s approved by the US Food and Drug Administration (FDA) to regulate blood sugar in people with type 2 diabetes, along with diet and exercise. 

It’s part of a drug class known as GLP-1s, which stands for glucagon-like peptide-1 receptor agonists. Ozempic, like the rest of the drugs in this class, does a few main things: 

• They bring down blood sugar (glucose) levels by increasing how much insulin the pancreas releases into the bloodstream when glucose levels spike.

• They lower appetite by slowing down digestion and sending fullness signals to the brain after you eat, which results in weight loss for many people. 

Is Ozempic approved for weight loss?

Even though Ozempic is effective at helping people lose weight (up to 15% of body weight on average), it’s not FDA-approved for weight loss. However, many providers prescribe it off-label for that purpose if they decide it’s the best treatment option for their patient. 

Semaglutide, the active ingredient in Ozempic, is also available as an FDA-approved weight loss drug under a different brand name, Wegovy.

How to use Ozempic

Ozempic is a drug you inject subcutaneously (just under the skin of your stomach, upper arm, or thigh) once a week. It’s meant to be taken long-term, since its effects start to wear off after about a week (hence, the weekly injection). Whether you’re using it for diabetes, weight management, or both, you’ll need to keep using Ozempic if you want to keep seeing any positive effects it has on your body. 

While some people feel skeptical about staying on a drug potentially for decades, it’s really no different from treatments for any other chronic condition. If someone has type 1 diabetes, they need to take insulin every day for the rest of their life (unless and until medical advancements give us a better solution). 

It’s the same thing when taking GLP-1s for weight loss (or diabetes). 

Unlike Ozempic, phentermine is FDA-approved as a weight loss drug. It’s an older medication, used to help with weight loss since 1959. Though the FDA only recommends it for short-term use (up to 12 weeks), many healthcare providers prescribe it off-label to their patients for longer than three months. It’s been proven safe and effective for long-term use for many people.

Phentermine is available as a generic drug, but can also be found under several brand names, including Adipex-P, Obenix, and Lonamin. 

There are those who feel a bit wary about phentermine since it’s a mild stimulant with some similarities to amphetamines, but it’s a much milder version and hasn’t been shown to have anywhere near the same potential for abuse. It does have a small level of abuse potential, though, so this is important to discuss with your healthcare provider. 

While phentermine is still commonly prescribed on its own, it’s now also available in a drug called Qsymia, which combines phentermine with topiramate. Qsymia is FDA-approved for the long-term treatment of obesity, along with lifestyle modifications.  

Does phentermine burn fat or just suppress appetite?

Phentermine works for weight loss by causing a surge of norepinephrine (a hormone and neurotransmitter) to release in the parts of the brain that control appetite. This effect suppresses appetite. 

There’s no evidence that phentermine burns fat directly, but if you pair this drug with exercise (preferably resistance training) while taking in fewer calories (made easier with a decreased appetite), you’ll likely see beneficial effects on body fat.  

How to use phentermine

While Ozempic is a weekly injectable drug, phentermine comes in tablet form, taken once daily by mouth. 

Since Ozempic and phentermine belong to different drug classes and work in distinct ways, it’s not surprising that their possible side effects differ. 

Ozempic side effects

The most common side effects of Ozempic include: 

• Nausea

• Vomiting 

• Diarrhea

• Stomach pain

• Constipation

You may notice a theme here—these common side effects are all related to gastrointestinal issues. This makes sense, since Ozempic affects how we digest food.

Phentermine side effects

While Ozempic’s side effects primarily relate to its impact on digestion, phentermine’s side effects are largely related to its being a mild stimulant. The most common side effects of phentermine include: 

• Nervousness and excitability

• Insomnia

• Headache

• Dry mouth or thirst

• Sweating

• Fast heart rate 

• Nausea

• Constipation 

These effects are similar to some of the effects of amphetamines, but much less severe.

When combined with a balanced diet and regular physical activity, phentermine and Ozempic’s active ingredient, semaglutide, are both effective for weight loss. 

Results vary by person, but on average, here’s what studies show: 

• Ozempic—One trial showed people taking 1.0 mg of semaglutide (along with lifestyle modifications) lost 7% of their weight on average, over 68 weeks. In another trial, people who took the highest dose of semaglutide (2.4 mg), along with lifestyle changes, lost an average of 15% of their body weight. 

• Phentermine—With phentermine, studies show that weight loss results depend on dose and how long someone takes the drug. Although it’s only FDA-approved for 12 weeks, studies demonstrate that taking it longer is better. Some research has shown that those who take a daily dose of 15 mg may see weight loss of up to 7% of their initial body weight after six months. 

Often, which one you and your provider choose will depend on other factors, like side effect tolerance, cost, or contraindications. 

Cost is one of the biggest variations between Ozempic and phentermine. 

Phentermine is available as a generic drug for around $30 per month out-of-pocket (depending on which pharmacy you use). 

Ozempic, on the other hand, can be quite costly if not covered by your insurance. The out-of-pocket cost can range from $800 to $1,200 monthly. Before you panic at that price, know there are ways to save money on Ozempic. 

For one thing, many insurance companies do cover this drug, especially if you have type 2 diabetes. If you don’t, and plan to use Ozempic for weight loss, your insurance might not cover it since it’s an off-label treatment. In that case, you can ask your insurance provider if they’ll cover Wegovy instead. It has the same active ingredient as Ozempic, but it’s FDA-approved for weight loss. 

Navigating insurance coverage can be tricky, but your healthcare provider’s office may have a coordinator on staff who can help you. Our Ro Body program includes insurance verification to take that burden off your shoulders.

The manufacturer of Ozempic, Novo Nordisk, also offers a savings program for eligible patients. 

Cost is a major factor in any decision, but it shouldn’t be your only deciding factor when choosing a weight loss medication. Working with your healthcare provider to find the right medication option for your circumstances is important. 

Ozempic and phentermine are safe for most people, but these drugs aren’t right for everyone. 

Ozempic contraindications

Don’t take Ozempic if you are nursing, pregnant, could become pregnant, or have a history of: 

• A certain type of thyroid cancer, called medullary thyroid carcinoma (MTC) 

• Multiple endocrine neoplasia type 2 (MEN 2), a rare cancer syndrome

You may need additional monitoring (or your provider may recommend a different drug) if you have a history of any of the following, as Ozempic could worsen these conditions or trigger an episode: 

• Pancreatitis

• Diabetic retinopathy

• Hypoglycemia

• Kidney disease

• Gallbladder disease

Phentermine contraindications

If you are pregnant, nursing, or have a history of any of the following, you shouldn’t take phentermine: 

• Heart disease (including high blood pressure, coronary artery disease, stroke, or congestive heart failure)

• Hyperthyroidism

• Glaucoma

• Drug abuse

Don’t take phentermine with alcohol or within 14 days of stopping an MAOI (monoamine oxidase inhibitor). 

Technically, you can take Ozempic and phentermine together, if your healthcare provider thinks that’s your best option. There are no known interactions between the two drugs so far. If you take two different medications for the same health goal, a healthcare provider usually starts you on one drug and slowly ramp up your dose as you can tolerate it. If you’re not seeing the amount of weight loss you and your provider have decided is achievable for you, they may recommend adding a second drug to help you get better results. Since semaglutide works very well for weight loss, it is quite likely that your healthcare provider decides against adding another drug, and increase the dose of semaglutide, instead. 

The more medications you take, the more chances there are for side effects. You may tolerate both drugs just fine, but let your provider know if you’re experiencing side effects that are too uncomfortable to tolerate. 

We’ve explored the similarities and differences between Ozempic and phentermine in terms of what they do, their side effects, how well they work for weight loss, cost, and contraindications. Take a look at how they stack up at-a-glance.

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Current version

May 30, 2023

Written by

Rachel Honeyman

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.