Ozempic vs. phentermine: Which weight loss medication is right for you?

Key Points

• Phentermine is a short-term appetite suppressant that works through the central nervous system, while Ozempic (semaglutide) is a GLP-1 agonist that slows digestion and reduces appetite.
• Ozempic leads to greater weight loss (15-20% of body weight) compared to phentermine (5-10% in the short term), but phentermine may work faster initially.
• Phentermine is typically prescribed for short-term use (up to 12 weeks), while Ozempic/Wegovy is approved for long-term weight management.

Are you considering weight loss medication? Wondering if phentermine is a better option than Ozempic or semaglutide? With so many options available, it can be confusing to understand which medication might work better for your specific needs.

While both medications can help with weight loss, they work differently in your body, have different side effects, and vary significantly in cost and how long you can take them. Let's break down the key differences to help you make an informed decision with your healthcare provider.

What is phentermine?

Phentermine is a prescription medication that's been around since 1959. It's an appetite suppressant that works by stimulating the central nervous system, essentially triggering your fight-or-flight response to reduce hunger signals.

If you've heard of "fen-phen," that was a combination of fenfluramine and phentermine that became popular in the 1990s but was later removed from the market because fenfluramine was linked to heart valve damage. Phentermine alone, however, has remained FDA-approved for weight loss.

Phentermine is available in the brand-name medications Adipex-P, Lomaira, and Suprenza.

How does phentermine work?

Phentermine works in your brain to reduce your appetite through stimulant effects. It's chemically similar to amphetamine and triggers the release of neurotransmitters that decrease your hunger signals, make you feel less interested in food, and create a feeling of fullness.

Because it acts as a stimulant, it also gives some people more energy, which might help them be more active throughout the day.

What is Ozempic?

Ozempic is a brand-name drug for semaglutide, which was originally approved to treat type 2 diabetes. While Ozempic itself is technically only FDA-approved for diabetes, a higher dose of semaglutide is marketed as Wegovy specifically for weight management.

Many people may be prescribed Ozempic "off-label" for weight loss, especially during times when Wegovy has been in short supply.

How does Ozempic work?

Ozempic works in a completely different way from phentermine. It's a GLP-1 receptor agonist, which means it mimics a hormone in your gut called glucagon-like peptide-1 (GLP-1).

By mimicking this hormone, Ozempic can:

• Slow down your digestion, so food stays in your stomach longer
• Send signals to your brain that you're full
• Reduce your appetite and food cravings
• Help regulate your blood sugar levels

Unlike phentermine, Ozempic is not a stimulant and doesn't work through your central nervous system.

Which leads to more weight loss, phentermine or Ozempic?

If you're considering these medications, you probably want to know which one helps people lose more weight. Here's what the research shows:

Phentermine weight loss results

In short-term studies (around 12 weeks), phentermine users typically lose about 5-10% of their body weight, or an average of 18 pounds (compared to just 3.7 pounds in placebo groups).

In one clinical trial, 95.8% of people taking phentermine lost at least 5% of their weight, and 62.5% achieved a 10% or greater weight loss over three months.

Ozempic weight loss results

Semaglutide (available as Ozempic or Wegovy) leads to significantly greater weight loss:

• Patients lose an average of 15% of their body weight (over a 68-week trial)
• Some lose as much as 20% of their body weight
• About 86% of people taking semaglutide lose at least 5% of their weight, and 50% lose 15% or more

For someone starting at 200 pounds, that's about 30-40 pounds lost with semaglutide compared to 10-20 pounds with phentermine.

Head-to-head comparison

While there haven't been direct studies comparing these two medications, meta-analyses examining multiple studies suggest semaglutide (Ozempic/Wegovy) produces significantly greater weight loss than phentermine.

One meta-analysis showed semaglutide users lost about 13.7% of body weight, compared to 9.1% for the best phentermine-containing therapy (phentermine-topiramate).

How quickly can you lose weight on phentermine vs. Ozempic?

When it comes to weight loss, a lot of people are concerned with how quickly they’ll start shedding pounds after starting a medication. This is one of the most notable differences between the two medications, with the effects of phentermine working much faster than Ozempic or semaglutide.

Phentermine works almost immediately to reduce appetite, with noticeable weight loss often starting within the first few weeks.

Ozempic takes longer to build up in your system, with dosing typically starting low and gradually increasing over 16-20 weeks to minimize side effects. Most Ozempic research was conducted over a year-long period or longer, showing that it’s not a quick fix, but a long-term weight loss solution.

While phentermine might show faster early weight loss, Ozempic is likely to surpass those results over time and yield more weight loss overall.

“The most success comes from slow and steady progress – about 1-2 pounds of weight loss weekly,” says Amy Bakios, RD. “We do what we can to maintain energy and satiety while avoiding malnutrition, making small changes that work for the long term. Before you know it, you will look back and realize you’ve formed new habits that are easy to maintain for life.”

Side effects: What to expect

Both medications can cause side effects, but the types of side effects differ significantly.

Phentermine side effects

Since phentermine is a stimulant, its side effects are similar to other stimulants. Common issues include dry mouth, insomnia and trouble sleeping, increased heart rate, nervousness or jitters, constipation, and elevated blood pressure in some people.

While these side effects can be bothersome, short-term studies show phentermine is generally well-tolerated in otherwise healthy adults. There are, however, many warnings that come with the drug, which we’ll get into later.

Ozempic side effects

Ozempic's side effects mainly affect your digestive system. The most common issue is nausea, which affects about 44% of patients. Other digestive problems include vomiting, diarrhea, constipation, and stomach pain. These side effects are usually at their worst when you first start taking the medication or when your dose increases, but they typically fade over time.

How long can you use phentermine & Ozempic?

One of the biggest differences between these medications is how long you can take them.

Phentermine duration

Phentermine is explicitly a short-term medication due to its stimulant properties.

• It is FDA-approved for short-term use only (up to 12 weeks)
• Many doctors limit prescriptions to 3 months at a time
• Some physicians may prescribe it for longer periods "off-label" if they believe the benefits outweigh the risks

The short-term limitation comes from a lack of long-term clinical trials and historical concerns about stimulant effects on the heart. However, newer data suggests that in carefully selected patients without heart problems, longer use might be safe.

Semaglutide duration

Semaglutide is a long-term medication, meant to be taken for at least a year or even indefinitely:

• Approved for chronic, long-term weight management
• Meant to be used indefinitely, similar to blood pressure or diabetes medication
• If you stop taking it, weight regain is common

It’s important to note that the weight loss and blood sugar effects of semaglutide only last as long as you’re taking the medication. Most people regain about two-thirds of the weight they lost within a year of stopping semaglutide.

Who should avoid each medication?

Not everyone is a good candidate for either of these medications.

Who should avoid phentermine?

Phentermine is not recommended for people with:

• Heart disease or history of heart problems
• Uncontrolled high blood pressure
• Hyperthyroidism
• Glaucoma
• History of drug abuse
• Pregnancy or breastfeeding
• Taking MAO inhibitors or certain antidepressants

The stimulant effects of phentermine can put extra strain on your heart, which is why it's contraindicated in people with cardiovascular issues.

Who should avoid Ozempic?

Semaglutide is safe for most people. However, those with a personal or family history of medullary thyroid carcinoma (MTC) and people with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2) can’t take it.

It's also not recommended for pregnant women, those with a history of pancreatitis in some cases, and individuals with severe gastrointestinal disease.

Cost & insurance coverage

The cost difference between these medications is dramatic. Phentermine is usually about $30 for a month's supply without insurance, while semaglutide is about $800-$1,000+ per month without insurance.

Insurance coverage varies widely:

• Most insurance plans don't cover phentermine, but since it's relatively affordable, many people pay out-of-pocket
• Some insurance plans cover Ozempic for diabetes but not for weight loss
• Wegovy may be covered for weight loss but often requires prior authorization and proof that you meet specific BMI criteria
• Medicare does not cover weight loss drugs by law, which affects coverage for both medications

According to economic analyses, phentermine (especially when combined with topiramate) is more cost-effective than semaglutide for weight loss.

Who should take phentermine vs. Ozempic?

Your personal health situation might make one medication a better fit than the other:

Diabetes consideration

If you have type 2 diabetes or prediabetes, Ozempic has clear advantages:

• Directly lowers blood sugar (can reduce A1C by 0.5-1.0%)
• Reduces risk of heart attacks and strokes
• Provides both diabetes management and weight loss in one medication

Phentermine doesn't directly improve blood sugar control, but weight loss itself can often help improve blood sugar levels.

Heart health consideration

Semaglutide has been shown to reduce risk of major cardiac events by 20% in people with existing heart disease.

Phentermine should be avoided in people with heart issues and has no evidence of cardiovascular benefit.

For those with normal blood sugar

If you don't have diabetes and have no heart issues, either medication could work for you, though phentermine might be preferred if cost is a major concern or you only need short-term help to jump-start weight loss. The side effects of Ozempic may also be too much to tolerate.

Can you take phentermine & Ozempic together?

Some people wonder if combining these medications might boost results. There's limited research on this specific combination, but a small pilot study that added phentermine to another GLP-1 medication (liraglutide) found:

• The combination led to only about 1-2% extra weight loss
• The small additional benefit disappeared after phentermine was stopped
• Side effects were more common with the combination

Currently, there's no strong evidence supporting the combination of these medications, and it's not standard practice.

What happens when you stop these medications?

Both medications seem to work only as long as you take them.

When people stop phentermine after 3 months, studies show they often return to baseline weight within 2 years if no other changes are maintained.

After stopping semaglutide, as mentioned, people typically regain about two-thirds of their lost weight within a year.

This highlights that obesity is a chronic condition, and ongoing treatment (whether medication or lifestyle changes) is usually needed for lasting results. It also underscores the importance of forming healthy habits while you’re on the medication so that the results can last longer, even if you do regain some of the weight you lost.

Final thoughts

Choosing between phentermine and Ozempic comes down to several factors:

• For maximum weight loss: Ozempic/Wegovy typically produces greater total weight loss
• For quick initial results: Phentermine might work faster in the short term
• For affordability: Phentermine is much less expensive
• For long-term use: Semaglutide is designed and approved for chronic use
• For diabetes or heart disease: Ozempic offers additional health benefits

Remember that both medications work best when combined with healthy eating habits and regular physical activity. Neither is a magic solution that works without lifestyle changes.

The choice between these medications should always be made with your healthcare provider, who can consider your medical history, current health status, and weight loss goals to determine the safest and most effective option for you.

Ready to find sustainable weight loss solutions? Work with a qualified dietitian to create a personalized plan that works for you and your goals. Find a weight loss dietitian who accepts your insurance with Fay.

Frequently Asked Questions

Will my insurance cover phentermine or Ozempic for weight loss?

Most insurance plans don't cover phentermine, though it's relatively affordable without insurance (about $30/month). For Ozempic/Wegovy, coverage varies widely. Some plans cover it for diabetes but not weight loss. Others may cover Wegovy for weight loss if you meet specific BMI criteria and have tried other weight loss methods first. Medicare does not cover weight loss medications by law.

How much weight can I expect to lose on each medication?

On phentermine, most people lose about 5-10% of their body weight during a 3-month period. With Ozempic/Wegovy, average weight loss is 15-20% of body weight over 68 weeks, with some people losing more and others less.

What happens if I get pregnant while taking these medications?

Both medications should be stopped if you become pregnant. Phentermine and semaglutide are not recommended during pregnancy, as they haven't been well-studied in pregnant women and could potentially harm the developing baby.

Can I drink alcohol while taking phentermine or Ozempic?

Alcohol can intensify some side effects of both medications. With phentermine, alcohol can increase cardiovascular side effects. With Ozempic, alcohol might worsen digestive symptoms and increase the risk of low blood sugar. It's best to limit or avoid alcohol while taking either medication.

Which medication is better if I have high blood pressure?

If you have well-controlled high blood pressure, Ozempic may be the safer choice as it doesn't raise blood pressure and might even help lower it slightly. Phentermine can raise blood pressure and heart rate in some people and is contraindicated if you have uncontrolled hypertension.

The views expressed by authors and contributors of such content are not endorsed or approved by Fay and are intended for informational purposes only. The content is reviewed by Fay only to confirm educational value and audience interest. You are encouraged to discuss any questions that you may have about your health with a healthcare provider.