Weight-loss injections, explained

A once-weekly shot that mimics a gut hormone is now the most effective non-surgical weight-loss treatment on record. What the drugs are, what the trials showed, and what to weigh first.

What they are and how they work

These drugs are synthetic versions of incretins — the gut hormones released after a meal. Semaglutide copies one (GLP-1), tirzepatide copies two (GLP-1 and GIP), and retatrutide, still in trials, adds glucagon. They dampen appetite in the brain, slow stomach emptying and improve insulin release, so people eat less without constant hunger. Doses start low and step up every four weeks.

What the trials showed

Average body-weight loss at the top dose in each drug's main trial, against roughly 2–3% for placebo.

Side effects, and what happens when you stop

The common effects are gastrointestinal — nausea, constipation, diarrhoea — and cluster around each dose increase. Rarer risks include gallbladder problems and pancreatitis; the drugs are not for anyone with a personal or family history of medullary thyroid cancer or MEN2, or who is pregnant. Appetite returns within weeks of stopping, and most of the lost weight comes back within a year, so clinicians treat these as long-term medications — making cost, supply and a prescriber who will monitor you the practical questions to settle first.

Research use only. A reading aid for the published literature, not medical advice. Talk to a licensed prescriber before starting or stopping any medication.