GLP-1 agonists started as single-receptor drugs. They act on the pancreas (insulin secretion), the gut (gastric emptying), and the brain (appetite and satiety). But the body has other metabolic networks. Each is a lock waiting for a key.
GIP is one of those keys. Its primary established action is potentiating insulin secretion from pancreatic beta cells. GIP also has receptors on fat tissue, and the role of GIP in adipose metabolism is an active research area. When tirzepatide activates both GLP-1 and GIP receptors, weight loss increases.
Glucagon is another. It tells the liver to burn stored energy. Retatrutide activates glucagon receptors too. The result is the highest weight loss seen in clinical trials. But these are newer pathways with thinner safety records. The body has never received sustained pharmacological glucagon receptor activation at these levels before.
This is where uncertainty lives. One peptide signal running for a year has decades of data behind it. Three signals running simultaneously for a year do not. We see the results clearly. The mechanisms we understand partially. The long-term effects we are still learning.
What happens when glucagon signaling runs continuously for five years? The compounds work. But they work in bodies that have never experienced this pattern before.