Peptide 101: Semaglutide — what it is, and what it actually does

Welcome to the first Peptide 101 — a recurring segment where we take a single peptide and explain it from the ground up. No hype, no hand-waving, no selling. Just what it is, how it works, what the evidence shows, and what to ask a clinician before it ever comes up.

We are starting with semaglutide for two reasons. It is the molecule that pulled this entire field into the headlines. And it is back in the regulatory news this week — which makes it the right place to begin.

What semaglutide actually is

Semaglutide is a GLP-1 receptor agonist. GLP-1 (glucagon-like peptide-1) is a hormone your gut already releases after you eat. Semaglutide is a manufactured molecule built to mimic it — but engineered to last for days instead of the few minutes your own GLP-1 survives.

It is sold under three brand names, and the difference between them matters more than most coverage admits:

• Ozempic — a weekly injection, FDA-approved in 2017 for type 2 diabetes.
• Wegovy — a weekly injection, FDA-approved in 2021 for chronic weight management in adults with obesity, or overweight with a related condition.
• Rybelsus — a daily oral tablet, FDA-approved in 2019 for type 2 diabetes.

Same molecule. Different doses, different formulations, different approved uses. “Ozempic for weight loss,” the phrase you have heard a hundred times, is technically off-label use of the diabetes product — the on-label weight product is Wegovy.

How it works

Here is the analogy we use: semaglutide amplifies your body’s natural “I’m full and satisfied” signal — the one that normally shows up after a good meal — and keeps it switched on for days instead of minutes.

Mechanically, it acts in several places at once:

• In the pancreas, it prompts insulin release, but mainly when blood sugar is actually high (which is why, on its own, it rarely causes dangerously low blood sugar). It also suppresses glucagon, a hormone that raises blood sugar.
• In the brain — the appetite centers of the hypothalamus and brainstem — it turns down hunger and the mental preoccupation with food.
• In the stomach, it slows how fast food empties, so fullness lasts longer.

That combination — less appetite, steadier blood sugar, slower digestion — is why one molecule ended up addressing both diabetes and obesity.

What the evidence actually shows

This is the part worth being precise about.

For weight: in the obesity trials behind Wegovy, average weight loss landed around 15% of body weight, sustained over time — a result no prior medication had matched.

For the heart: the SUSTAIN-6 trial in people with type 2 diabetes found semaglutide reduced major cardiovascular events — heart attack, stroke, cardiovascular death — by roughly 26%. Cardiovascular benefit was later also shown in people with obesity who did not have diabetes. That shift matters: it moved semaglutide from “a drug that improves lab numbers” to “a drug shown to change hard outcomes.”

The limits, stated plainly:

• The benefits depend on continued use. When people stop, appetite and much of the weight tend to return. This behaves like a long-term medication, not a one-time course.
• A long list of other uses — Alzheimer’s prevention, addiction, PCOS, sleep apnea, kidney and liver disease — are under active study but not FDA-approved. Early signals are not the same as proof, and we will treat them that way here.

Safety — the honest version

The most common side effects are gastrointestinal: nausea, vomiting, diarrhea, and constipation, usually worst while the dose is being increased. That is why prescribers raise the dose slowly over weeks rather than starting high.

Two things deserve real attention:

• Semaglutide carries an FDA boxed warning for a risk of thyroid C-cell tumors seen in rodent studies. It is contraindicated for anyone with a personal or family history of medullary thyroid carcinoma, or the genetic syndrome MEN2.
• Less common but serious risks include pancreatitis and gallbladder disease, and — in people with type 2 diabetes who already have eye disease — a possible worsening of diabetic retinopathy. (That last signal is one researchers are still actively investigating; see below.)

This is not a complete safety list, and none of it is dosing advice. It is the set of things worth knowing before a conversation with a prescriber.

Questions worth bringing to a clinician

If semaglutide is on the table for you or someone you know, these travel well:

• Which formulation and indication actually fits my situation — diabetes, weight, or cardiovascular risk?
• What is the plan if I stop — how do we keep the benefits, or transition off?
• What should we monitor, and how often?

This week in peptides: the 503B notice landed

On May 1, the FDA’s formal notice published in the Federal Register. It proposes not to add semaglutide, tirzepatide, and liraglutide to the 503B “clinical need” bulks list — the regulatory step we walked through in our May 18 letter on the 503B exclusion proposal. In plain terms: the path to large-scale compounding of these GLP-1s through outsourcing facilities is being formally closed now that the shortages are over. If you read the May 18 piece, this is the paperwork catching up to the policy.

New research, translated

A 2026 review in the Indian Journal of Pharmacology gathered one-to-two-year clinical-trial evidence for semaglutide in obesity and type 2 diabetes. Worth knowing what this is and isn’t: a review consolidates existing trials — it is a useful map of the evidence, not new evidence itself. Its value is in showing how consistent the findings are across studies, rather than any single headline.

Separately, a new systematic review examined a possible link between semaglutide and a rare eye condition (non-arteritic anterior ischemic optic neuropathy). This is an association under investigation, not an established cause and effect — exactly the kind of early signal that deserves attention without alarm.

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That is semaglutide from the ground up. Next week’s Peptide 101 takes on a very different molecule — one you cannot pick up at any pharmacy.

— The Editors