How GLP-1 Receptor Agonists Drive Weight Loss
GLP-1 agonists mimic the natural gut hormone glucagon-like peptide 1. They slow gastric emptying, suppress appetite via the hypothalamus, and improve insulin sensitivity. The result for most patients is dramatically reduced food intake without willpower fatigue — typically 20–35% fewer daily calories without conscious restriction. This calculator is free to use and all projections run entirely in your browser.
Worked Example: 100 kg on Wegovy 2.4 mg
Using the calculator's default figures — a 100 kg starting weight at 170 cm tall, on Wegovy (semaglutide 2.4 mg) at the trial-average response — the projected trajectory is:
| Timepoint | Projected weight | Lost so far |
|---|---|---|
| 3 months | 95.5 kg | −4.5 kg |
| 6 months | 91.1 kg | −8.9 kg |
| 12 months | 87.3 kg | −12.7 kg |
| 18 months (plateau) | 85.1 kg | −14.9 kg (−14.9%) |
That leaves an estimated final BMI of about 29.4. Change the medication, dose and adherence profile above to see how the projection shifts — for example, tirzepatide 15 mg at the trial average projects roughly a 22.5% loss over the same period.
What the Clinical Trials Show
- STEP-1 (Wegovy 2.4mg): mean −14.9% body weight at 68 weeks vs −2.4% placebo
- STEP-UP (Wegovy HD 7.2mg, FDA approved Mar 2026): mean ~−18% at 72 weeks
- SURMOUNT-1 (Zepbound 15mg): mean −22.5% at 72 weeks; 57% of patients lost ≥20%
- SCALE (Saxenda 3.0mg): mean −8% at 56 weeks
- Retatrutide Phase 2: mean −24% at 48 weeks (still in development)
Loss Trajectory
Loss is non-linear. About 30% of total loss happens by month 3, 60% by month 6, 85% by month 12, with most patients plateauing between months 15–18. The plateau occurs because metabolic adaptation reduces TDEE and the drug's anorectic effect partially offsets at lower body weight.
Maintenance & Discontinuation
STEP-4 showed that switching from active drug to placebo at week 20 caused patients to regain about ⅔ of lost weight within the next year. Current evidence suggests GLP-1s are best viewed as long-term therapy for chronic obesity, not a short course. Lifestyle work (resistance training, high-protein diet) makes maintenance after eventual discontinuation more durable.
Side Effects
Most common: nausea (~40%), constipation, fatigue, decreased appetite — usually worst in the first 4–8 weeks of dose escalation and typically improves with slower titration. Rare but serious: pancreatitis, gallstones, gastroparesis, kidney injury via dehydration. Black-box warning for thyroid C-cell tumours (MTC) — contraindicated if personal or family history of MTC or MEN2.
Protect Your Lean Mass
Roughly 25–40% of GLP-1 weight loss is lean tissue — higher than diet alone. To minimise muscle loss while on GLP-1s:
- Hit 1.6–2.2 g/kg protein per day (use our Protein Intake Calculator)
- Perform resistance training 2–3× per week
- Avoid aggressive caloric deficits — let the drug do the work
- Track body composition (DEXA, BIA) every 3–6 months, not just scale weight
Frequently Asked Questions
How much weight can I lose on Wegovy or Ozempic? In the STEP-1 trial, semaglutide 2.4 mg (Wegovy) produced a mean ~14.9% body weight loss at 68 weeks vs ~2.4% with placebo. Ozempic 1 mg (a lower dose with a type 2 diabetes indication) typically yields ~6–7%. Individual response varies — about a third of patients are "super-responders" losing 20%+, while about 15% lose under 5%.
How does Mounjaro / Zepbound compare? Tirzepatide (Mounjaro for diabetes, Zepbound for weight loss) is a dual GIP+GLP-1 agonist. SURMOUNT-1 showed a mean ~22.5% weight loss at 15 mg over 72 weeks — roughly 50% more than semaglutide. Wegovy HD 7.2 mg narrows the gap to ~18%. Tirzepatide remains the most effective approved GLP-1 medication for weight loss.
What is the typical weight loss timeline? Loss usually starts within 4 weeks. Most patients reach about 30% of their total loss by month 3, 60% by month 6, 85% by month 12, and plateau by month 15–18. Speed depends on starting weight, dose, adherence, diet and exercise. Stopping the drug generally leads to about two-thirds of the lost weight returning within a year (STEP-4 extension data).
Do I lose muscle on GLP-1s? Body composition studies show roughly 25–40% of weight lost on GLP-1s is lean tissue — higher than diet alone, which is typically 20–25%. To minimise muscle loss, aim for 1.6–2.2 g/kg of protein daily, perform resistance training 2–3 times per week, and avoid aggressive deficits.
Can I take a GLP-1 if I am not diabetic? Yes — Wegovy, Zepbound and Saxenda are licensed specifically for weight management in people without diabetes, typically at a BMI of 30+, or 27+ with a weight-related condition such as high blood pressure or sleep apnoea. Ozempic and Mounjaro are licensed for type 2 diabetes but are sometimes prescribed off-label for weight loss. Eligibility and licensing vary by country and prescriber.
How much do GLP-1 weight loss medications cost? Out-of-pocket costs vary widely by country and pharmacy, but branded GLP-1s commonly run from roughly £150–£250 per month in the UK to $1,000+ per month in the US without insurance. Prices change frequently, and insurance, NHS criteria and manufacturer savings programmes can change what you actually pay. This calculator does not estimate cost — check current pricing with a licensed pharmacy.
Is this calculator a substitute for medical advice? No. This is an educational tool using published clinical trial averages. GLP-1 medications are prescription drugs with side effects, contraindications (personal or family history of medullary thyroid carcinoma, MEN2, pancreatitis) and interactions. Always work with a licensed prescriber and never source these drugs from unregulated suppliers.
Last reviewed: June 2026. Educational estimates only — based on published clinical trial averages, not a prediction for any individual. Consult a licensed clinician before starting, changing or stopping any medication.