How does Mounjaro work for weight loss?

Mounjaro (tirzepatide) is FDA-approved to help people with type 2 diabetes manage their blood sugar levels, but it’s shown remarkable potential for weight loss. So much so that the FDA fast-tracked approval of tirzepatide for the treatment of obesity, which was granted in October of 2023 under the brand name Zepbound. We’ll dive into how Mounjaro works, how quickly you can expect to lose weight on Mounjaro, and just how much weight you can expect to lose.

Mounjaro is a lot like other popular medications used for weight loss and diabetes management, including Ozempic, Wegovy, and Saxenda. These medications all work by slowing the transit of food through the digestive system and making you feel full for longer. But unlike these drugs, which only affect the GLP-1 (glucagon-like peptide-1 receptor), Mounjaro and its weight-loss-approved counterpart, Zepbound, also affect a second receptor known as GIP (glucose-dependent insulinotropic polypeptide receptor). 

If you’re taking Mounjaro, you’ll inject it once a week subcutaneously (just under the skin on the stomach, thigh, or upper arm) with a tiny needle that is relatively painless for most people.  

Even though Mounjaro isn’t technically approved as a weight loss drug, many healthcare providers prescribe it off-label to their patients for that purpose because it’s proven so effective (skip to the next section to learn just how effective it can be). Off-label prescribing is common in healthcare, and in the case of Mounjaro, the active ingredient in the exact same available doses is approved for promoting weight loss in people with obesity. Still, a healthcare provider may choose to prescribe Mounjaro instead, for example, in order to ensure availability during a shortage, or if a person also has diabetes in order to improve the chances of insurance coverage.

So, what's the secret behind Mounjaro's success at aiding with weight loss? It all comes down to how it acts on receptors in the body. Mounjaro binds to two key receptors: the GLP-1 receptor and the GIP receptor. It mimics the actions of two naturally occurring hormones that bind those receptors that cause a cascade of effects when we eat. 

Often called incretin hormones, GLP-1 and GIP are involved in regulating how much insulin the pancreas releases into the bloodstream after a meal, how quickly the stomach empties itself of food, and how your brain regulates your appetite, among other things. These hormonal effects naturally kick into gear when we eat, but without Mounjaro, these effects last only for a few minutes.

When you introduce Mounjaro into your system, it binds to GLP-1 and GIP receptors but stays active for about a week (hence, those weekly shots). This prolonged activation means your body can better manage blood sugar levels and control your appetite, ultimately leading to weight loss.

Weight loss with Mounjaro is significant: the active ingredient in the drug, tirzepatide, seems to be the most effective drug in its class on the market for weight loss––more effective than Ozempic or Wegovy. Clinical studies showed that more than half of the participants who took the highest dose of Mounjaro lost over 20% of their body weight. The only other weight loss intervention that offers such dramatic results is bariatric surgery. 

Like any medication, results vary from person to person and are dose-dependent. Your healthcare provider will start you on a lower dose and likely slowly increase the amount of medication you get each week in order to minimize side effects, and this approach can change how quickly you lose weight.

*In a 72-week clinical trial studying Zepbound in patients without diabetes and with BMI ≥30, or BMI ≥27 with a weight-related condition, the average weight loss was 20% in people who took 15 mg, when paired with diet and exercise changes (compared to 3.1% with diet and exercise alone) and 15% in people who took 5 mg (the highest dose available in Zepbound single-dose vials).

While it can take up to a year or more to see the full effects of Mounjaro, many people begin losing weight within a few weeks of starting the drug. 

One large study showed an average of about 6% weight loss by week 8. That average percentage continued to increase throughout the 72-week study period. You can expect more dramatic results with a higher dose, but know that side effects are also dose-dependent; the dose your provider recommends will depend on how you’re tolerating the side effects.

Like any medical intervention, Mounjaro comes with its own set of considerations. While the side effects are generally manageable, they can include gastrointestinal symptoms, such as nausea, diarrhea, bloating, and decreased appetite. 

Your healthcare provider will start you on a low dose and gradually increase your dose every four weeks, as long as you’re tolerating the side effects. Everyone responds to these side effects differently. Some people may experience mild stomach discomfort right when they start the medication, with side effects decreasing over time. For a small percentage of people, Mounjaro’s side effects may be intolerable, or they may only be tolerable at lower doses. 

It's essential to work with your healthcare provider to develop strategies to mitigate any discomfort, and to discuss your medical and family history in detail, since Mounjaro is not recommended for people with certain medical conditions.

Mounjaro and Zepbound are actually the same medication: they both contain the active ingredient tirzepatide and both are available in the same dosages. So what’s the difference? The difference is that while the brand name Mounjaro is FDA-approved to treat type 2 diabetes, the brand name Zepbound is FDA-approved to help with weight loss. This distinction might seem irrelevant to the average patient but comes into play when it comes to insurance companies. 

Licensed healthcare providers can prescribe medications off-label (i.e. they could prescribe Mounjaro for weight management even to people who don’t have type 2 diabetes) but insurance companies are less likely to cover the medication in that case. That’s why the approval of tirzepatide for weight management is a blockbuster for patients. It will likely increase patient access to the medication for weight management purposes.

If you’re still considering whether a weight loss medication is right for you, you’re probably wondering how Mounjaro compares to the other big players on the market: Ozempic and Wegovy. 

While all three medications are highly effective at encouraging weight loss, Mounjaro has emerged as a standout player, boasting a unique advantage. Like Mounjaro, Ozempic and Wegovy belong to the GLP-1 receptor agonist family, but Mounjaro brings something extra to the table: its GIP receptor activation. 

This dynamic duo of receptor activation sets Mounjaro apart and makes it even more effective at encouraging weight loss, along with lifestyle changes.

Semaglutide, the active ingredient in both Ozempic and Wegovy, has been shown to promote an average of 15% weight loss at the highest dose for some people. That makes Wegovy the most effective FDA-approved weight loss drug on the market (Ozempic, like Mounjaro, is currently only approved as a diabetes drug, though it’s often prescribed off-label for weight loss). 

Still, as we’ve already seen, Mounjaro comes out ahead in studies, resulting in over 20% reduction in body weight for the majority of study participants. 

If your healthcare provider decides that Mounjaro is right for you, you can expect to see some modest weight loss pretty quickly—within a month or so, with more results over time. After being on the drug for a year or more, you may see 15–20% weight loss from your starting weight, though results can vary from person to person and depend on what dosage you’re taking. 

Side effects tend to be the worst at the start of any new medication regimen, as your body is getting used to the new drug. So, don’t fret if you have pretty uncomfortable side effects right when you start taking Mounjaro—you may just need to give it some more time. 

Once you start taking Mounjaro, you’ll need to stay on it long-term if you want to keep seeing results. It may be difficult to imagine staying on a medication long-term, but it’s important to remember that it’s no different than any other drug you need to take for any other condition. Obesity is a chronic condition, and if you need medical help to manage it, it’s just like taking insulin every day if you have insulin-dependent diabetes. 

Also, any successful weight loss intervention will only work long-term if you keep it up. If you’ve ever successfully lost weight through another method, like diet and exercise, you know that as soon as you stopped those efforts, you regained the weight. It’s the same with Mounjaro. 

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Current version

November 29, 2023

Written by

Rachel Honeyman

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.