Semaglutide.

The drug that made peptides a phenomenon.

Semaglutide is the drug that turned the peptide industry from a quiet specialty into a cultural phenomenon. It is a once-weekly injection (or daily tablet) that mimics the body's own glucagon-like peptide-1, a hormone released by the gut after eating.

It produces dramatic improvements in blood sugar control in type 2 diabetes and substantial, durable weight loss in people with obesity. In its first five years on the market, it grew into one of the highest-selling pharmaceutical products of all time, and it has reshaped the pipelines of every major drug company.

How it actually works.

GLP-1 is an incretin hormone released by L-cells in the small intestine after a meal. When it binds the GLP-1 receptor on pancreatic beta cells, it stimulates glucose-dependent insulin secretion — the cells release more insulin when blood sugar is high, and less when it is normal. It also suppresses glucagon, the hormone that raises blood sugar between meals.

Beyond the pancreas, GLP-1 receptors are present in the brain (where activation reduces appetite and food reward signaling), in the stomach (where activation slows gastric emptying), and in the heart and kidneys (with effects on inflammation and natriuresis). Semaglutide hits all of these sites because its long half-life keeps the receptor activated continuously rather than only in the post-meal window.

The result is a coordinated change in how the body handles food: less appetite, smaller portions, slower stomach emptying that prolongs fullness, better insulin response to whatever is eaten, and reduced glucagon-driven liver glucose output.

What the FDA approved it for.

• Type 2 diabetes mellitus — to improve glycemic control, with reductions in HbA1c typically in the 1.5 to 2.0 percentage-point range.
• Chronic weight management — in adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity, and in adolescents 12 and older with obesity. Mean weight loss in the STEP trials was approximately 15% of body weight at 68 weeks at the 2.4 mg weekly dose.
• Cardiovascular risk reduction — to reduce major adverse cardiovascular events in adults with type 2 diabetes and established cardiovascular disease (the SELECT and SUSTAIN-6 outcome trials).

What doctors are prescribing it for anyway.

• Weight management below the BMI threshold — individuals seeking to lose 10 to 20 pounds for aesthetic rather than medical reasons. This is the largest single source of off-label use and the driver of the cultural moment around the drug.
• Polycystic ovary syndrome (PCOS) — improvements in weight, insulin resistance, and menstrual regularity are reported in small trials and routinely in clinical practice.
• Metabolic-dysfunction-associated steatohepatitis (MASH) — tirzepatide and survodutide are further along in this indication, but semaglutide is used off-label for fatty liver disease, supported by a Phase 2 trial showing histological improvement.
• Substance-use cravings — early signals in animal and small human studies suggest the drug may reduce alcohol craving and nicotine use. Active area of research, not yet evidence-based clinical practice.
• Cognitive protection and Alzheimer's prevention — a large outcome trial (EVOKE) is ongoing in mild cognitive impairment. Off-label prescribing for this purpose is currently premature.

What it does well.

• Substantial, durable weight loss at the higher doses — approximately 15% of body weight on average, with about a third of patients losing 20% or more.
• Major improvements in HbA1c, often allowing patients to reduce or stop other diabetes medications.
• Cardiovascular benefit — meaningful reduction in heart attack, stroke, and cardiovascular death in patients with established cardiovascular disease.
• Metabolic improvements — reductions in blood pressure, triglycerides, and inflammatory markers.
• Once-weekly dosing that most patients find easier than daily injections.
• Emerging signals of benefit in fatty liver disease, kidney protection in diabetes, and reduction of craving behaviors.

What can go wrong.

• Gastrointestinal side effects — nausea, vomiting, constipation, and diarrhea are common, especially during dose escalation, and cause about 5 to 10% of patients to discontinue.
• Loss of lean body mass — like any rapid weight loss, semaglutide-induced weight loss includes some loss of muscle. Resistance training and adequate protein intake reduce but do not eliminate this.
• Gastroparesis — delayed gastric emptying that becomes symptomatic. Usually resolves on discontinuation but can be debilitating.
• Pancreatitis — a rare but serious risk; the drug should be stopped permanently if pancreatitis is suspected.
• Gallbladder events — increased risk of gallstones, especially with rapid weight loss.
• Black-box thyroid warning — risk of medullary thyroid carcinoma seen in rodents; relevance to humans is uncertain. Contraindicated in people with a personal or family history of medullary thyroid cancer or MEN2.
• Weight regain after discontinuation — without ongoing dosing or sustained lifestyle change, two-thirds of weight lost is typically regained within a year.
• Cost and access — list price is high, insurance coverage is variable for weight management indications, and shortages have repeatedly affected supply.
• "Ozempic face" — rapid weight loss can produce a hollowed appearance in the face and neck; not a drug-specific effect but a consequence of any rapid weight loss in adults.

How strong is the science?

The takeaway.