GLP-1 Weight Loss Drugs: How They Work, Results and Risks

GLP-1 receptor agonists have changed obesity treatment more than anything in decades: for the first time, drug therapy delivers weight loss in the same order of magnitude as bariatric surgery. Along with that has come a cloud of mythology, from "magic injection" to "terrifying chemicals". Here is what the trials show, what to expect realistically, and what to know before starting.

How they work#

GLP-1 (glucagon-like peptide-1) is a hormone released by intestinal cells in response to food. It enhances insulin secretion when blood glucose is high, suppresses glucagon, slows gastric emptying and acts on satiety centres in the hypothalamus and brainstem. These drugs are degradation-resistant analogues given once weekly.

Subjectively, people describe the effect as the disappearance of "food noise" — intrusive thoughts about eating. Portions shrink naturally, cravings for energy-dense food fall, and fullness arrives sooner and lasts longer. That is a fundamental difference from older weight-loss agents, which worked by stimulating the nervous system.

Tirzepatide acts on two receptors — GLP-1 and GIP — producing a stronger effect. Retatrutide and other next-generation agents are in trials.

Drug	Mechanism	Average weight loss	Trial
Liraglutide 3.0 mg	GLP-1 agonist, daily	about 8 % over 56 weeks	SCALE
Semaglutide 2.4 mg	GLP-1 agonist, weekly	about 15 % over 68 weeks	STEP 1
Tirzepatide 15 mg	Dual GIP/GLP-1 agonist	up to 20.9 % over 72 weeks	SURMOUNT-1

What the key trials showed#

STEP 1 (2021) enrolled 1961 participants without diabetes and a BMI of 30 or above. At 68 weeks, average weight loss was 14.9 % versus 2.4 % on placebo. More than a third of participants lost over 20 % of body weight, with improvements in waist circumference, blood pressure, lipids and inflammatory markers.

SURMOUNT-1 (2022) tested tirzepatide in 2539 participants. At 15 mg, average loss reached 20.9 % over 72 weeks, with more than half of participants losing at least 20 %.

SELECT (2023) is the pivotal hard-outcomes trial: over 17,000 people with obesity and established cardiovascular disease but no diabetes. Semaglutide reduced cardiovascular death, heart attack and stroke by roughly 20 %. That moved these drugs from the "weight loss" category into the category of treatments that change prognosis.

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Side effects#

Effect	Frequency	What to do
Nausea	Very common (up to 40–50 %)	Slower dose escalation, smaller portions, less fat
Vomiting, diarrhoea, constipation	Common	Usually eases with time; if persistent, review the dose
Belching, reflux	Common	Don't lie down after eating, smaller frequent meals
Gallstones	Uncommon	Related to rapid weight loss itself
Pancreatitis	Rare	Severe pain radiating to the back — see a doctor immediately
Dehydration, kidney impairment	Rare	Drink enough, especially with vomiting
Gastroparesis, bowel obstruction	Rare	Persistent vomiting or distension — see a doctor

Contraindications include a personal or family history of medullary thyroid carcinoma, multiple endocrine neoplasia type 2, a history of pancreatitis (relative), and pregnancy or planning pregnancy. These drugs slow gastric emptying, which matters when preparing for anaesthesia — always tell your anaesthetist you are taking one.

Muscle, skin and "Ozempic face"#

Any substantial weight loss removes more than fat. Lean mass accounts for roughly 25–40 % of the weight lost on GLP-1 therapy — comparable to dietary weight loss of similar magnitude, though the absolute figures are larger because total loss is larger.

That imposes a requirement that is often ignored:

• Protein at 1.2–1.6 g per kg of target body weight per day. With reduced appetite this takes planning: protein first, vegetables and fats after.
• Resistance training 2–3 times a week — the only proven way to preserve muscle in a deficit.
• Watch for deficiencies. Sharply reduced food volume puts iron, B12, calcium and vitamin D at risk.

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So-called "Ozempic face" is not a drug-specific effect but the consequence of rapid loss of facial subcutaneous fat, which occurs with any fast weight loss. The same applies to changes in skin quality.

Who they are for#

Regulatory indications in most regions: BMI of 30 or above, or BMI 27 or above with at least one weight-related condition — hypertension, dyslipidaemia, prediabetes, type 2 diabetes, sleep apnea, osteoarthritis. The decision belongs to a clinician who assesses contraindications, concurrent medication and readiness for long-term management.

Context matters here: obesity is a chronic disease with strong biological defence of established body weight. The body responds to weight loss by lowering energy expenditure and raising ghrelin, and these drugs partly counteract exactly those mechanisms. Hence the logic of long-term rather than course-based use.

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The bottom line#

GLP-1 agonists are the most effective obesity pharmacotherapy available today: about 15 % weight loss with semaglutide and up to 21 % with tirzepatide, plus a 20 % reduction in cardiovascular events in SELECT. Side effects are mainly gastrointestinal and track with escalation speed. The key limitations: about two thirds of the weight returns after stopping, and muscle is lost as it is on any diet — so protein and strength training are non-negotiable. These are prescription medicines for people with obesity or weight-related complications, not body-shaping tools, and a clinician should be the one prescribing them.