An engineered hormone analogue
Semaglutide's fatty branch
The branch helps a peptide hold on to a blood protein. The substitutions help it resist cutting. Together, they explain a striking change in duration.
Semaglutide substitutions
Semaglutide is a 31-amino-acid analogue of GLP-1 with about 94% sequence similarity. The substitution at position 8 replaces alanine with alpha-aminoisobutyric acid and resists cutting by the DPP-4 enzyme, which rapidly breaks down natural GLP-1. The substitution at position 34 replaces lysine with arginine; the lysine at position 26 carries the fatty side chain. Those position numbers follow the parent hormone's numbering.
The attached fat binds albumin reversibly. While attached, semaglutide is protected from rapid removal; some can detach and act. Together with resistance to the DPP-4 enzyme, this supports a half-life of about a week, compared with about two minutes for natural GLP-1. The safety review introduces the molecule's prolonged action.
I think the reversible part is particularly elegant. A molecule can spend time protected and still become available to its receptor. That is a chemical explanation of duration. Its value as a medicine needed separate experiments.
Appetite and gastric emptying
Activating GLP-1 receptors increases insulin secretion when blood sugar is elevated and reduces inappropriate glucagon release. Semaglutide also slows stomach emptying. Its weight effect comes chiefly through brain circuits governing appetite and fullness, reducing food intake rather than directly increasing calorie burning. The clinical safety review discusses these effects and their trade-offs.
That stomach effect has an awkward companion: nausea. The biology sounds less frictionless once appetite and tolerability are considered together. That combination of useful and uncomfortable effects deserves a full account.
Semaglutide trial results
In STEP 1, involving 1,961 adults with overweight or obesity without diabetes, mean body-weight change at 68 weeks was -14.9% with semaglutide and -2.4% with placebo. Both are changes from starting weight.
SELECT enrolled 17,604 adults with established cardiovascular disease and overweight or obesity, without diabetes. Semaglutide reduced the combined risk of cardiovascular death, nonfatal heart attack or nonfatal stroke by 20% relative to placebo. FLOW studied 3,533 adults with type 2 diabetes and chronic kidney disease, finding a 24% lower relative risk for the composite of kidney failure, major kidney-function decline or kidney/cardiovascular death. Relative risk reductions compare groups; they aren't percentage-point changes in the number affected.
The SURMOUNT-5 comparison with tirzepatide asks a different question again. I value the organ-outcome studies because the endpoints reach beyond the scale, while their entry criteria make clear whose outcomes were measured.
Prescription status and safety
Semaglutide has FDA-approved prescription uses for type 2 diabetes, chronic weight management and reduction of major cardiovascular events in adults with established cardiovascular disease and overweight or obesity. Approval also covers MASH, the inflammatory fatty-liver disease, since 2025. Approval applies to the relevant manufactured medicine and indication; research or compounded material sits outside that approved-product evidence.
The safety review describes predominantly mild-to-moderate gastrointestinal effects and increased gallbladder disease. The boxed warning concerns thyroid C-cell tumours in rodents. Personal or family history of medullary thyroid carcinoma or MEN-2 is a contraindication; a clear human thyroid-cancer signal has not been established. Acute pancreatitis carries a class warning. Pregnancy rules out use, and individual medical decisions require a licensed prescriber.
Personal accounts describe quieter ‘food noise’, changed food preferences and weight loss, as well as nausea, constipation, diarrhoea, sulfur-smelling burps and fatigue. These are anecdotal accounts rather than clinical evidence for those individual experiences. Even with a well-studied medicine, a vivid account cannot show who else will have the same experience. The FAQ returns to that difference when mixtures and certificates enter the conversation.