1The Signal Line

You finish a meal. As it moves into the small intestine, specialized cells in the intestinal lining, called L-cells, respond to it.

They release a hormone called GLP-1, short for glucagon-like peptide-1, into the bloodstream.

GLP-1 carries one message to three places: the pancreas, the nerves that run the stomach, and the brain.

The message is roughly “enough.” Release insulin for the sugar that’s arriving. Slow the stomach down. Ease off the drive to eat.

Every drug on this line activates the same receptor GLP-1 does. What changes is how strongly, and for how long.

GLP-1 doesn’t last. An enzyme in the blood called DPP-4 breaks it down within minutes.

So after each meal, GLP-1 rises sharply and falls back almost as quickly. Across a day, the receptor gets a few brief pulses.

The first GLP-1 drug, exenatide (sold as Byetta), had a half-life of about 2.4 hours: half of each dose was gone in that time. Patients injected it twice a day.

Later drugs were engineered to linger. Semaglutide (Ozempic, Wegovy) carries a fatty chain that anchors it to albumin, a common blood protein, so the kidneys clear it slowly.

A weekly injection keeps the receptor activated across all seven days. Drug levels peak and then drift down before the next dose, but the receptor stays activated the whole week.

It is also a much stronger signal. A therapeutic dose activates the receptor far more than any meal does, and that sustained, high-level activation is where the large weight loss comes from.

In the pancreas, small clusters of cells called islets manage blood sugar. Beta cells release insulin, which lowers it. Alpha cells release glucagon, which raises it.

GLP-1 tells beta cells to release insulin in proportion to the glucose just absorbed. More sugar arriving means more insulin.

When blood sugar isn’t elevated, the extra push on insulin largely fades. That is why these drugs, used on their own, carry a low risk of driving blood sugar too low.

GLP-1 also suppresses glucagon, the hormone that tells the liver to raise blood sugar.

At diabetes doses of semaglutide (Ozempic, 0.5 to 2 mg weekly), this blood sugar effect dominates, and weight loss is modest.

The biggest contribution to weight loss comes from the brain. GLP-1 receptors sit in the hypothalamus, the region that pulls together the body’s fullness signals.

Activating them strengthens the feeling of fullness after eating.

Receptors in the brain’s reward regions (the nucleus accumbens and the ventral tegmental area) add something more. They reduce the drive to eat, beyond simply feeling full.

Many people on these drugs describe it as quieter “food noise”: fewer intrusive thoughts about eating.

This is where dose matters. At the obesity dose (semaglutide 2.4 mg weekly, sold as Wegovy), appetite effects dominate. Average weight loss in the pivotal trial was about 17% of body weight, roughly one pound in six. A higher 7.2 mg dose (Wegovy HD, approved in the US in March 2026) reached about 21% in its trial.

Both figures are the average weight loss in the semaglutide group, without subtracting the placebo group’s loss. Both use the efficacy estimand, which asks what would have happened if everyone had stayed on treatment. It is not the same as averaging only the people who finished. The treatment-policy estimand, which counts everyone assigned whether or not they stopped, gives figures a few percentage points lower.

GLP-1 also slows the stomach. Food leaves it for the small intestine more slowly than usual.

It works through nerves, not the stomach muscle itself. Receptors on the vagus nerve and on the gut’s own network of nerves set the slower pace.

A slower stomach adds to the feeling of fullness after a meal.

It also lets sugar from a meal reach the blood more gradually, which blunts the spike after eating.

It helps, but it’s a supporting act. Most of the weight effect still comes from the brain.

Nausea is the most common side effect. A primary reason is the area postrema, a spot in the brainstem that triggers vomiting. It has GLP-1 receptors too.

Nausea, vomiting, constipation, and diarrhea are the common ones, and they depend on dose.

So treatment starts low. The dose steps up about every four weeks, taking 16 to 20 weeks to reach the full dose.

Most people feel side effects most sharply in the first weeks after each step up. Then they ease.

Skipping steps makes for a rougher ride. That is part of why side effects in everyday practice often look worse than in clinical trials.

GLP-1 receptors show up far beyond the pancreas, the gut’s nerves, and the brain. So researchers tested these drugs on the heart, the kidneys, the liver, and more.

Heart: in the SELECT trial of more than 17,000 people with obesity and heart disease but no diabetes, semaglutide cut major cardiovascular events by about 20%. That is more than the weight loss alone would predict.

Kidneys: the FLOW trial, in people with type 2 diabetes and kidney disease, stopped early because the benefit was clear, with about 24% fewer serious kidney outcomes.

Liver: in August 2025, the FDA approved semaglutide for MASH, a progressive fatty liver disease, based on the ESSENCE trial.

Brain: the least settled. In alcohol use disorder, a 2026 trial of 108 people who also had obesity found semaglutide cut heavy drinking days by about 14 percentage points more than placebo over 26 weeks, on top of therapy both groups received. Trials in Parkinson’s and Alzheimer’s disease came back negative.

A signal that lasts minutes after a meal turns out to reach across the body. Other lines will show what changes when a drug adds GIP, glucagon, or amylin.