Mounarjo vs Phentermine for Weight Loss

Explore the emerging landscape of prescription weight loss medications, including GLP-1 agonists and alternatives like phentermine. Learn the differences between Mounjaro (trizepatide) and phentermine, assisting in making informed decisions aligned with your health goals.

Table of Contents
  • Medications That Cause Weight Gain:
  • Summary
  • What Can You Do About Medication-Related Weight Gain?

Mechanism of Action

Mounjaro is a novel medication that works by combining three different hormones into one drug. These hormones include GLP-1, glucagon, and GIP, and they work together to control blood sugar levels and reduce appetite. GLP-1 is a hormone that is naturally produced in the body and helps to regulate glucose metabolism and appetite. Glucagon is another hormone that helps to regulate glucose metabolism, while GIP is a hormone that helps to stimulate insulin secretion from the pancreas.

Phentermine works by suppressing appetite. It is a sympathomimetic amine and acts as a central nervous system stimulant, which increases the release of neurotransmitters such as norepinephrine, dopamine, and serotonin in the brain. These neurotransmitters help to reduce hunger and increase feelings of fullness, which can lead to decreased calorie intake and weight loss.

Dosage and Administration

Mounjaro is administered once a week as a subcutaneous injection (under the skin). The recommended starting dose is 2.5 mg once a week, which can be increased up to 18 mg once a week if necessary.

Phentermine is usually taken orally in the form of a tablet or capsule. The usual recommended dose is 37.5 mg once a day, although the dose may be lower in some cases.


Overall, both medications are effective for weight loss in conjunction with diet and exercise. 

For Mounjaro, a trial published in 2022 looked at how effective Mounjaro (tirzepatide) was at managing obesity . Mounjaro was given at different doses for a period of 72 weeks to groups of non-diabetic participants who were struggling with obesity. The participants were also enrolled in a calorie restriction diet (500 kcal/day), did 150 minutes of exercise per week, and had regularly scheduled with a dietician. 

They found that after 72 weeks, each group that was taking Mounjaro showed significant weight loss compared to the placebo group. Each treatment group experienced a 15-20% change in weight from the start of the trial, with higher doses showing a greater change. This was compared to the placebo group, which only experienced a 3% change in weight. This showed us that the use of Mounjaro in combination with diet and exercise is very effective at treating obesity.

Phentermine has been used as a short term weight loss medication for over a decade. In a randomized control trial published in 2006 on the short term effects on weight loss in korean patients struggling with obesity, researches found that partipants taking 37.5 mg of Phentermine noticed a significant drop in mean body weight and waist circumference compared to placebo. The study showed that, in combination with diet and exercise, phentermine was effective in weight management however the study was only done over 14 weeks. Approximately 86% of the participants experienced a 5% reduction in body weight or greater and about 50% experienced greater that 10% reduction in body weight. 

Both studies show that these are effective medication at managing obesity, however an edge goes to Mounjaro since it was study over 72 weeks compared to the 14 weeks for phentermine. This may indicated that mounjaro is more effective in the long term. 

Side Effects

Both Mounjaro and phentermine can cause side effects, although the type and severity of these side effects can vary.

Mounjaro has been associated with gastrointestinal side effects such as nausea, vomiting, and diarrhea. However, these side effects tend to be mild to moderate in severity and usually improve over time. Mounjaro can also increase the risk of hypoglycemia (low blood sugar) and pancreatitis, although these side effects are rare. Mounjaro should not be taken if you have a history of medulary thyroid cancer or a family history of multiple endocrine neoplasia cancer syndrome type 2 (MEN-2). It should also be avoided if you are taking insulin for type 2 diabetes or if you have a history of diabetic retinopathy. 

Phentermine can cause side effects such as dry mouth, constipation, and insomnia. It can also increase blood pressure and heart rate, which may be a concern for people with cardiovascular disease or other health conditions. Phentermine has the potential for abuse and dependence, and it should be used with caution in people with a history of substance abuse.

Cost and Coverage

Mounjaro is currently not approved by the FDA for the management of obesity, but this may change in the near future. Overall, it is expected to be a more expensive medication than phentermine, which is available in generic form and is relatively inexpensive. It may be covered if you do have diabetes and would like to gain the weight loss benefits of the medication while managing blood sugar.


In conclusion, Mounjaro and phentermine are two medications that are used for weight loss, but they work in very different ways and have different benefits and side effects. Mounjaro appears to be more effective than phentermine for weight loss in the long term, but is a lot more expensive. Phentermine is a relatively inexpensive medication that can effectively treat obesity in the short term, however there is a risk for abuse and dependence. In the end, a detailed discussion should be had with your provider to determine the pros and cons of each medication and which one will work best for you. It should be noted that these medications should be taken in conjunction with diet and exercise in order to achieve the best results. 

Where Can I Find These Medications?

If you would like to learn more about GLP-1 medications or want to see if you might be eligible for medication-assisted weight loss, check out Mochi Health, where board-certified obesity medicine physicians can offer expertise in this realm!

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