GLP-1 Peptides: Semaglutide, Tirzepatide, Retatrutide & How They Work

How GLP-1 Peptides Work

GLP-1 receptors are found in the pancreas, brain (hypothalamus, brainstem), stomach, heart, kidneys, and other tissues. Activating these receptors produces multiple coordinated effects:

• Appetite suppression (hypothalamus): GLP-1 acts directly on hunger centers in the brain to reduce "food noise" — the persistent mental preoccupation with eating that drives overconsumption. Clinical trial participants frequently describe this as the most significant subjective effect.
• Slowed gastric emptying: GLP-1 delays how fast food moves from the stomach to the small intestine, extending satiety and blunting post-meal glucose spikes.
• Glucose-dependent insulin release (pancreas): GLP-1 stimulates insulin secretion only when blood glucose is elevated, eliminating hypoglycemia risk (unlike sulfonylureas).
• Glucagon suppression: Reduces the liver's glucose output by suppressing glucagon, further stabilizing blood sugar.
• Cardiovascular protection: GLP-1 receptors in the heart and vessels produce anti-inflammatory effects. The SELECT trial showed semaglutide reduced major cardiac events by 20% in non-diabetic individuals with existing cardiovascular disease.

GLP-1 Drug Comparison

Semaglutide vs. Tirzepatide: Which to Choose

Both are effective — tirzepatide produces roughly 50% more weight loss on average. The key differences:

• Efficacy: Tirzepatide's dual GIP/GLP-1 mechanism appears synergistic. GIP receptors are expressed in fat cells directly, adding a fat-mobilizing effect that pure GLP-1 agonists don't have. Average weight loss is significantly higher.
• Side effects: Similar GI side effect profiles (nausea, diarrhea, constipation during titration). Some clinicians report tirzepatide causes slightly less nausea, possibly because GIP receptor activation partially counteracts GLP-1's nausea-inducing effects at the brainstem.
• Cost: Similar list price (~$1,000/month without insurance). Coverage varies significantly.
• Availability: Both have been subject to supply shortages and compounding pharmacy access issues. Check current status with your prescriber.

Comparing specific brands or alternatives? See Ozempic vs Wegovy (same drug, different use), Ozempic vs metformin, berberine vs Ozempic, and the head-to-head drug comparison: Ozempic vs Mounjaro or the obesity-approval showdown at Wegovy vs Zepbound.

What's Coming: The Next Generation

Retatrutide (Eli Lilly's triple agonist) showed 24.2% average weight loss at 48 weeks in Phase II — a figure previously seen only with bariatric surgery. Phase III data is expected 2025–2026. If confirmed, it may become the highest-efficacy pharmacological weight-loss option available without surgery.

Oral non-peptide GLP-1 agonists (orforglipron, danuglipron) are in Phase III and could dramatically expand access — eliminating the injection requirement that remains a barrier for some patients. These are small molecules, not peptides, but they activate the same GLP-1 receptor.