Tirzepatide (Mounjaro and Zepbound) is the first U.S. Food and Drug Administration (FDA)–approved drug that is part of a new drug class called glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor agonists.
Tirzapetide is administered either as a once-weekly injection under the skin of your abdomen, thigh, or upper arm using a prefilled pen or as a pill, according to the manufacturer. Doses range from 2.5 milligrams (mg) to 15 mg.
“Mounjaro has to be titrated when it is started, meaning a gradual dose increase happens over many months, to allow the body to adjust to the medication and lower the risk of side effects,” says Florencia Halperin, MD, a Boston-based endocrinologist who is the chief medical officer of the weight loss clinic Form Health.
The Mechanism Behind GLP-1 and GIP Receptor Agonists
Not all of the mechanisms of this class of drugs are known yet. What is known is that the medications mimic the actions of two incretin hormones: GLP-1 (which is in the upper gut) and GIP (which is in the lower gut). The duo works together to stimulate insulin secretion after a meal is eaten, according to research and the American Diabetes Association.
The insulin then helps your body use or store glucose (sugar) in a way that the body can’t on its own due to type 2 diabetes. With type 2 diabetes, the body can’t handle insulin as efficiently as it should, and you can develop high blood sugar, or hyperglycemia.
The drug affects multiple organs in the body, mimicking and stimulating both the GIP and GLP-1 receptors in our bodies, Dr. Halperin says.
“These receptors are found in the pancreas, where stimulation leads to increased insulin secretion. They are also found in the appetite centers of the brain. GLP-1 also slows down stomach emptying, so people feel fuller sooner and longer. And GIP is known to act on our fat cells in a variety of ways,” Halperin says.
What’s the Difference Between Tirzepatide and Semaglutide?
The drug may sound similar to another type 2 diabetes and weight loss drug, semaglutide, which is a GLP-1 receptor agonist. Similar to tirzepatide, semaglutide — sold as the type 2 diabetes drug Ozempic and the obesity drug Wegovy — mimics the hormone GLP-1 and triggers the body to make more insulin when blood sugar levels rise after eating. GLP-1 receptor agonists are so popular that they were on the FDA’s shortage list in 2023, but have since been taken off.
Tirzepatide is different from semaglutide because it also mimics the GIP hormone. Meaning, it affects not only stomach emptying and appetite but also how fat is deposited in the body.
What Is Tirzepatide Approved to Treat?
In May 2022, the FDA approved Mounjaro for type 2 diabetes, and five months later, the agency gave it a fast-track designation to expedite the review of results from two phase 3 clinical trials investigating it as a weight loss treatment, according to Eli Lilly. Results from the second of those trials were released by Eli Lilly on April 27, 2023, and subsequently published online in The Lancet in June 2023.
In November 2023, the FDA approved tirzepatide for weight loss under the name Zepbound. Research has shown that tirzepatide helped people lose an average of 27 percent of their body weight over the course of a year and a half, when combined with diet and exercise.
Tirzepatide is not indicated for people with type 1 diabetes, according to the manufacturer. However, a recent 12-week clinical trial found that the medication was superior to placebo for weight loss in adults with type 1 diabetes. While promising, additional studies are needed.
Only your healthcare team can determine if these meds are a good fit for you, so be sure to consult them to discuss what’s right for your health and wellness goals.
