In this guide
How the signal works
GLP-1 stands for glucagon-like peptide-1. It is a hormone involved in appetite and food intake. A receptor is a part of a cell that receives a signal. Semaglutide activates the GLP-1 receptor. Tirzepatide activates this receptor and a second one called GIP. GIP stands for glucose-dependent insulinotropic polypeptide, another hormone involved in metabolic signals.[1][2]
The labels describe effects on brain regions involved in appetite. Their direct evidence of nerve-cell activation comes from animal studies. Human trials provide a different kind of evidence: reported hunger and the amount people eat.[1][2][3]
What studies measured in people
A 20-week trial studied 72 adults with obesity. Participants received injected semaglutide, increased to 2.4 mg once weekly, or placebo, a treatment without the active medicine. At a test lunch, the semaglutide group ate 35% fewer calories than the placebo group. They also reported less hunger and greater fullness.[3]
That figure describes a meal measured under study conditions. It does not mean everyone eats 35% less each day.
A later trial followed 120 adults with overweight or obesity for 60 weeks. It compared semaglutide 2.4 mg with placebo. The semaglutide group had larger reductions in calories eaten at test lunches at weeks 20, 40, and 60. However, differences in reported appetite were no longer statistically significant at weeks 40 and 60.[4]
Reported hunger and measured food intake can change differently. These group results cannot determine whether a medicine is working for an individual. The later study also reports author relationships with drug companies.[4]
Slower stomach emptying is part of the explanation
Gastric emptying means food moving from the stomach into the small intestine. The semaglutide label says the medicine delays this process. For tirzepatide, the label says the delay is largest after the first dose and decreases over time.[1][2]
The 20-week semaglutide trial found no evidence of delayed emptying at its final assessment. It used an indirect test based on absorption of paracetamol. This finding is limited to that method and time point. It does not cancel the product warning about delayed emptying.[3]
Both product labels warn about stomach and bowel side effects that can be severe. Slower emptying is not proof of safe treatment.[1][2]
Effects on blood sugar are separate
Semaglutide and tirzepatide also affect insulin and glucagon, hormones involved in blood sugar control. Their labels describe insulin release that depends on glucose levels and reduced glucagon release.[1][2]
Those effects do not make the products interchangeable. A mechanism describes how a medicine acts. It does not settle which product, if any, is suitable for a person.
What this explanation cannot tell you
Weight-management medicines differ in their uses, benefits, and risks. A clinician considers health conditions and other medicines when assessing treatment. Understanding the mechanism alone does not provide that assessment.[5]
Product identity also matters. The FDA says unapproved GLP-1 products do not undergo its review for safety, effectiveness, and quality. It also identifies semaglutide sodium and acetate as different active ingredients from those in approved products. A familiar ingredient name does not establish equivalence.[6]
Common questions
Does feeling less full mean a medicine has stopped working?
Reported hunger and measured food intake can change differently. In the 60-week semaglutide trial, lower test-meal intake persisted even when group differences in appetite ratings were no longer statistically significant. Those group results cannot determine whether a medicine is working for an individual.