GLP-1 Medications Explained: How They Work

GLP-1 medicines — the class behind Ozempic, Wegovy, Mounjaro, and Zepbound — work on a hormone your body already makes. Understanding that one fact makes almost everything else about these drugs click.

What does GLP-1 stand for?

GLP-1 means glucagon-like peptide-1, a hormone released by cells in your gut after you eat. It does a few jobs: it signals your brain that you're full, slows how fast your stomach empties, tells your pancreas to release insulin when blood sugar is high, and tells it to back off glucagon (the hormone that raises blood sugar) when it's low.

The problem is that your natural GLP-1 is broken down in seconds by an enzyme called DPP-4. The medicines in this class are engineered versions that resist that breakdown, so their effects last hours to days instead of moments.

How do GLP-1s cause weight loss?

The appetite effect is the headline. In clinical trials, people on these medicines report feeling fuller and thinking less about food between meals. That reduced calorie intake — not a magic metabolic switch — is the main driver of the weight loss.

• Fullness and reduced food urges — the primary mechanism for weight loss.
• Slower stomach emptying — contributes to the sustained fullness.
• Better blood-sugar control — the reason the same molecules are also used for type 2 diabetes.
• Modest effects on the heart and kidneys — the subject of large outcome trials (see our Research section).

What is the difference between semaglutide and tirzepatide?

Semaglutide mimics a single hormone. Tirzepatide mimics two — GLP-1 and GIP — which is why it tends to show slightly more weight loss in head-to-head comparisons. Both are once-weekly injections in their branded forms.

Can you still get compounded GLP-1s?

This changed in 2025. When branded supply was short, telehealth services could offer compounded semaglutide and tirzepatide under a shortage exemption. The FDA declared both shortages resolved — tirzepatide in December 2024, semaglutide in February 2025 — and the deadlines to stop compounding under that exemption passed during 2025. Routine, mass-market compounding of these molecules is no longer permitted on that basis.

A narrower path remains. Under section 503A, a licensed pharmacy can still compound a patient-specific preparation against a prescription when the prescriber documents a clinically significant difference from the approved product for that particular patient. That is a real but limited exception — it is not the same as a subscription service shipping a standard dose to anyone who fills in a form. Adding something like vitamin B12 does not by itself make a formulation stop being essentially a copy.