What is GLP-1? A beginner's guide.

A receptor agonist is a molecule shaped to fit a receptor and switch it on — a key cut to match a lock the body already owns. GLP-1 receptor agonists are engineered versions of the natural hormone, altered at a few positions so the DPP-4 enzyme can't dismantle them, with a fatty-acid chain added so the molecule binds albumin and circulates for days. The result is a signal the body recognizes, sustained for roughly a week per injection instead of minutes per meal.

Semaglutide — the molecule sold as Ozempic for type 2 diabetes and as Wegovy for chronic weight management — is the most widely prescribed GLP-1 receptor agonist. Tirzepatide, sold as Mounjaro and Zepbound, takes the design one step further: it activates the GLP-1 receptor and a second incretin receptor, GIP, a pairing that produced greater average weight reduction in head-to-head trials.

The medication signal is also deliberately stronger than physiology. Where your own GLP-1 arrives in brief pulses after meals, the engineered versions hold the receptor active continuously, at higher-than-physiologic levels. That difference — duration and intensity, not kind — is why the medications change appetite in a way the natural hormone never quite could.

Appetite is the lever. With GLP-1 receptors held active, satiety arrives earlier in a meal and lasts longer after it. Many patients describe a quieting of 'food noise' — the constant background thinking about the next meal — within days of the first injection. Eating less stops being an act of willpower and becomes the path of least resistance.

Slowed gastric emptying does the supporting work. Food leaves the stomach gradually, so fullness stretches across the hours between meals instead of collapsing into the next craving. The same mechanism produces the class's familiar side effects — nausea, early fullness, constipation — which cluster in the weeks after each dose increase and settle as the body adapts.

In trials, that adds up to roughly 15 percent average body-weight reduction on semaglutide and up to about 21 percent on tirzepatide over a year-plus of full-dose therapy. What these medications don't do matters as much: they don't speed metabolism, dissolve fat, or burn calories. They make sustained caloric reduction achievable — the physiology still does the arithmetic.

The FDA's weight-management labels set the starting thresholds: a BMI of 30 or higher, or 27 with a weight-related comorbidity such as type 2 diabetes, hypertension, or sleep apnea. We treat those numbers as the opening of a clinical conversation, not the whole of one — history, prior attempts, metabolic context, and goals all belong in it.

Screening rules some patients out. We don't prescribe with a personal or family history of medullary thyroid carcinoma or MEN-2, a history of pancreatitis, or during pregnancy or attempts to conceive. And we don't prescribe GLP-1 therapy for cosmetic weight reduction at a normal BMI — the bar for off-label use is high, and the screening is the same either way.

For candidates, therapy lives inside a supervised program: baseline labs, a deliberately low starting dose, titration decisions every four weeks, and monitoring that continues through maintenance. A hormone with this much reach deserves that structure — which is most of what separates a program from a prescription.