If you’re struggling with obesity, you might have tried many ways to lose weight. Eating less bread, eating more greens, and taking the stairs are all common tips. But these tips often don’t work because obesity is not just about habits. It’s a chronic disease that affects your body’s metabolism.

While exercise and healthy eating are important, they might not be enough for everyone. Many people are now looking at medications to help with weight loss. You’ve probably heard about popular medications like Mounjaro and Phentermine. Both are used by many to lose weight, but how do they compare in terms of effectiveness?

In this article, we will compare Mounjaro and Phentermine to see which one is approved by the FDA and proven to be more effective for weight loss. We will look at how they work, their side effects, and what studies say about their effectiveness.

Key Takeaways

  • Mounjaro is a GLP-1 receptor agonist primarily used to improve glycemic control in adults with type 2 diabetes, while Phentermine is a sympathomimetic amine anorectic approved as a short-term adjunct for weight reduction in patients with exogenous obesity.
  • Mounjaro is administered as a subcutaneous injection once weekly, with a dosage that may be increased over time. Phentermine is typically taken in capsule form, with a typical adult dose ranging from 15 mg to 30 mg taken approximately 2 hours after breakfast.
  • Mounjaro’s common side effects include gastrointestinal issues and potential hypersensitivity reactions, along with the risk of hypoglycemia when used with insulin or insulin secretagogues. In contrast, Phentermine’s side effects include increased heart rate, dry mouth, sleeplessness, nervousness, and constipation.
  • Mounjaro targets GLP-1 and glucose-dependent insulinotropic polypeptides to promote a feeling of fullness, lower blood sugar, and stimulate insulin release, thereby facilitating weight loss. Phentermine, a sympathomimetic amine, primarily works for weight loss by suppressing appetite and increasing energy levels.
  • Although not officially approved by the FDA as a weight-loss medication, Mounjaro has shown effectiveness in reducing body weight, leading many doctors to prescribe it for this purpose. Phentermine is specifically indicated for short-term use as an adjunct in a regimen of weight reduction based on exercise, behavioral modifications, and caloric restriction.
  • Mounjaro is approved for use in adults, whereas Phentermine is not recommended in individuals under 16.

Comparing Mounjaro vs Phentermine

Drug ClassGlucose-dependent insulinotropic polypeptide (GIP) receptor and glucagon-like peptide-1 (GLP-1) receptor agonistSympathomimetic amines anorectic
Indications and UsesMounjaro is indicated as an adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes mellitus.It is indicated as a short-term adjunct for weight reduction in patients with exogenous obesity, with a body mass index (BMI) ?30 kg/m² or ?27 kg/m² with other risk factors like hypertension or diabetes.
Age ApprovalApproved for use in adultsNot recommended for use in individuals under 16 years of age.
Dosage And AdministrationInitial dosage: 2.5 mg injected subcutaneously once weekly. Dosage may be increased over time following specific guidelines.The typical adult dose ranges from 15 mg to 30 mg taken approximately 2 hours after breakfast.
Forms and StrengthsInjection is available in pre-filled single-dose pens in strengths of 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg.Available in capsule form, containing 15 mg or 30 mg of Phentermine Hydrochloride.
Side EffectsCommon side effects include gastrointestinal issues, hypoglycemia when used with insulin or insulin secretagogues, and potential hypersensitivity reactions.Side effects can include increased heart rate, dry mouth, sleeplessness, nervousness, and constipation.

How Does Mounjaro Work for Weight Loss

How Does Mounjaro Work for Weight Loss

Mounjaro, known by its generic name tirzepatide, is a prescription medication initially approved to treat Type 2 diabetes but is also used off-label for weight loss in obese individuals. It targets two hormones: GLP-1, which promotes a feeling of fullness and helps lower blood sugar, and glucose-dependent insulinotropic polypeptide, which stimulates insulin release. 

This dual action not only assists in managing diabetes but also alters how the body reacts to food, leading to increased satiety and reduced calorie intake, thereby facilitating significant weight loss. Although not officially approved as a weight-loss medication by the FDA, its effectiveness in reducing body weight has led many doctors to prescribe it.

Mounjaro Dose for Weight Loss

Mounjaro is typically 2.5 milligrams (mg) injected once weekly for the first four weeks. Following this initial period, the dosage is generally increased by 2.5 mg every four weeks until an effective dose for weight loss is reached. The exact dosage can vary based on individual response to the medication. 

Although Mounjaro is not officially approved for weight loss, some patients have reported successful weight loss results when using the drug in conjunction with a balanced diet and exercise. It’s important to consult with a healthcare provider to discuss the potential benefits and risks, as well as to determine the appropriate dosage for personal health goals and conditions.

Side Effects of Taking Mounjaro for Weight Loss

According to a study, the side effects of taking Mounjaro (tirzepatide) for weight loss, especially in patients with type 2 diabetes mellitus (T2DM) and obesity, include a range of gastrointestinal issues and other adverse events. These may include:

  • Diarrhea
  • Nausea
  • Vomiting
  • Decreased appetite
  • Constipation
  • Hypoglycemia
  • Injection Site Reactions

How Does Phentermine Work for Weight Loss

Phentermine primarily works for weight loss by suppressing appetite and increasing energy levels. These effects are mediated through its action as a sympathomimetic amine. The medication stimulates the release of norepinephrine in the brain, particularly affecting the hypothalamus, an area involved in appetite control. By activating the central nervous system in this way, Phentermine helps reduce hunger, increase satiety, and elevate the user’s energy expenditure, all of which contribute to weight loss. 

This medication is typically used as a short-term adjunct in a regimen of weight reduction based on exercise, behavioral modifications, and caloric restriction, specifically for patients with a significant initial body mass index and associated risk factors such as hypertension, diabetes, or hyperlipidemia.

Phentermine Dose for Weight Loss

For optimal results, it should be combined with a reduced-calorie diet and exercise. The typical dosage varies depending on the form of phentermine: adults and children aged 17 and older can take 15 to 30 milligrams (mg) once a day of the capsule form about 2 hours after breakfast. Alternatively, one tablet can be taken once a day in the morning if the disintegrating tablets are used. 

For the extended-release capsules, the recommended dose for adults and children aged 16 and older is one capsule daily, taken before breakfast or 10 to 14 hours before bedtime. Lastly, the tablet form, specifically Lomaira, is dosed with one tablet thrice daily, 30 minutes before meals. 

It is important to note that the use of phentermine in children younger than 16 is not recommended, and dosages may be adjusted by a healthcare provider based on individual needs and response to the medication.

Side Effects of Phentermine

The use of phentermine may lead to various side effects in addition to weight loss. These may include

  • Insomnia (when taken late in the evening)
  • Primary Pulmonary Hypertension (PPH), is a rare but potentially fatal lung disease
  • Serious regurgitant cardiac valvular disease, primarily affecting the mitral, aortic, and/or tricuspid valves
  • Development of tolerance to the drug’s appetite-suppressing effects
  • Risk of abuse and dependence, similar to that of amphetamines
  • Possible adverse reactions when used with alcohol
  • Increased blood pressure in patients with hypertension

How Effective Are Mounjaro vs Phentermine for Weight Loss?


Clinical trials have reported that individuals taking Mounjaro can lose a substantial amount of weight compared to those on placebo or other diabetes medications. Participants taking Mounjaro experienced an average weight loss of approximately 9.81 kilograms (21.6 pounds) more than those on a placebo. Higher doses of Mounjaro resulted in greater weight loss. For example:

  • At a 5 mg dose, the average weight loss was about 7.52 kilograms (16.6 pounds).
  • At a 10 mg dose, the average weight loss increased to approximately 10.48 kilograms (23.1 pounds).

Another study published in the New England Journal of Medicine conducted a large trial involving 2539 adults with obesity. Tirzepatide (Mounjaro)  was administered once weekly in varying doses of 5 mg, 10 mg, and 15 mg. The results showed significant weight reductions: average reductions were 19.5% for the 10 mg dose and 20.9% for the 15 mg dose over 72 weeks, compared to just a 3.1% reduction with a placebo. 

Additionally, most participants receiving the 10 mg and 15 mg doses achieved weight loss of more than 5%, which is considered clinically meaningful for improving metabolic health. The treatment was also associated with improvements in cardiovascular and metabolic risk factors.

These results demonstrate Mounjaro’s considerable effectiveness in aiding weight reduction, with higher doses yielding more significant outcomes.


According to a study, approximately 45.6% of the individuals who took phentermine lost at least 5% of their body weight. This study involved 795 people, and out of those who were part of the effectiveness check, 324 individuals achieved this level of weight loss. The average weight lost was about 3.8 kilograms, indicating that phentermine can be quite effective in helping people lose weight, especially in the short term.

Another study indicates that individuals who take phentermine for more than 12 weeks, longer than the usual recommendation, tend to lose more weight. Specifically, long-term users can lose about 7.4% more body weight than those who don’t use it for as long. Even after one year, these users typically maintain a significant amount of weight loss. 

However, the effectiveness can vary, with some people experiencing minimal weight loss. The study also noted that using phentermine did not increase the risk of cardiovascular disease or death, suggesting it could be a safe option for extended use in certain low-risk individuals.

Which is More Affordable – Mounjaro or Phentermine?

In the US, those without coverage for Mounjaro can save up to $573 off their 1-month prescription fill, with a maximum monthly savings of $573 and a maximum annual savings of $3,438 per calendar year. Alternatively, Buy Canadian Insulin offers Mounjaro at a more affordable starting price of only $158.99 per vial. By using the coupon code ‘FIRST10’, customers can save an additional 10% on their first order. 

In comparison, the cost of Phentermine in the US varies: for 37.5mg tablets, the price ranges from $14.82 to $17.31 for 30 tablets and $21.62 to $22.96 for 45 tablets. For 37.5mg capsules, the price ranges from $6.97 to $17.55 for 30 capsules, and for 30mg capsules, the price ranges from $5.61 to $19.89 for 30 capsules.

Final Thoughts

When comparing Mounjaro and Phentermine for weight loss, both medications have shown effectiveness in aiding weight reduction. Mounjaro, based on clinical trials, has demonstrated substantial weight loss with higher doses, resulting in greater outcomes and improvements in cardiovascular and metabolic risk factors. 

On the other hand, Phentermine has also proven to be effective, with some individuals experiencing significant weight loss, especially with long-term use, without an increased risk of cardiovascular disease or death. While Mounjaro appears to yield more significant weight loss based on the available data, Phentermine also shows promise as a weight loss medication, particularly for those who may benefit from extended use.

FAQs About Mounjaro and Phentermine

What’s better, Mounjaro or phentermine?

Comparing Mounjaro and phentermine depends on individual health needs. Mounjaro is a GLP-1/GIP receptor agonist primarily for diabetes and potential weight loss, while phentermine is an appetite suppressant used for short-term weight management.

What drug is comparable to Mounjaro?

Zepbound (tirzepatide) is a drug comparable to Mounjaro for weight loss. Both drugs contain the same active ingredient, tirzepatide, and are FDA-approved for weight management and their use for diabetes management.

What are the best alternatives to Phentermine?

Some alternatives to phentermine for weight loss include prescription GLP-1 agonists like Ozempic, Wegovy, Saxenda, and Zepbound, which help control appetite and promote weight loss. Over-the-counter supplements like PhenQ and Zotrim use natural ingredients to support fat burning and reduce cravings.

Can you take Mounjaro and Phentermine together?

Taking Mounjaro (tirzepatide) and phentermine together is not generally recommended without consulting a healthcare provider. Combining them could increase the risk of side effects such as increased heart rate, high blood pressure, and other cardiovascular issues. A healthcare professional can evaluate your situation and determine the best and safest approach for weight management. Always follow medical advice when considering medication combinations.

How to take phentermine 37.5 for best results?

Phentermine 37.5 is typically taken once daily before breakfast or 1-2 hours after breakfast. Follow your doctor’s instructions for the best results.

Does phentermine burn fat or just suppress appetite?

Phentermine primarily works by suppressing appetite, which can lead to reduced calorie intake and subsequent fat loss. However, it does not directly burn fat. Phentermine is most effective for weight loss when combined with a calorie-controlled diet and regular exercise to create a calorie deficit and promote fat-burning.

Why am I not losing weight on Mounjaro?

Not losing weight on Mounjaro could be due to various factors, such as diet, exercise, individual response, or underlying health conditions. Consult your healthcare provider for personalized advice.

Can I take phentermine 37.5 mg twice a day?

Taking phentermine 37.5 mg twice a day is not recommended. The standard dosage is one 37.5 mg tablet taken once daily in the morning. Taking it twice daily could increase the risk of side effects like insomnia, dry mouth, and increased heart rate without providing additional weight loss benefits. Always follow your doctor’s instructions when taking phentermine.

How quickly do you lose weight on Mounjaro?

In clinical trials, participants taking Mounjaro lost an average of 15-22% of their body weight over 72 weeks. However, individual results may vary. Some people may start seeing modest weight loss within the first few weeks, but maximum weight loss typically occurs after several months of consistent use. Combining Mounjaro with a healthy diet and exercise plan can help optimize weight loss results.

How quickly does Mounjaro suppress appetite?

Mounjaro can start suppressing appetite within the first few days of use. By activating GLP-1 and GIP receptors in the brain and gut, Mounjaro signals that you’re full, reducing hunger and cravings. This appetite suppression and slower gastric emptying can lead to reduced calorie intake and subsequent weight loss over time. However, the degree and duration of appetite suppression may vary between individuals.

How long can you take phentermine for weight loss?

Phentermine is approved for short-term use (a few weeks to a few months) for weight loss. The maximum recommended duration is 12 weeks, as its effectiveness for weight loss may decrease after this time. Prolonged use can also increase the risk of side effects and dependence. If you need to continue using phentermine for weight loss, it’s best to take breaks and consult your doctor regularly to monitor your progress and safety.

Do doctors still prescribe phentermine?

Yes, doctors still prescribe phentermine as an appetite suppressant for short-term weight loss when other methods have not been effective.

How quickly do you lose weight on phentermine?

Weight loss on phentermine varies, but it is typically noticeable within a few weeks of starting treatment, alongside diet and exercise modifications.

What is better than phentermine for weight loss?

Several alternatives are considered more effective than phentermine for weight loss, particularly for long-term results. Medications like Zepbound (tirzepatide),  liraglutide (brand name Saxenda), and semaglutide (brand names Ozempic and Wegovy) have also shown significant success.


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