Ask how do GLP-1 drugs cause weight loss and the one-line answer is: mainly by making you eat less. Mounjaro, Wegovy, the Wegovy pill and Foundayo all switch on the receptor for GLP-1, a gut hormone the body uses to regulate appetite. Keep that receptor active for days at a time and hunger falls, meals feel more filling and cravings weaken, so calorie intake drops; a slower stomach and steadier blood sugar play supporting roles.
Information only, not medical advice: all four are prescription-only, and a prescriber decides suitability. For the overview, see what GLP-1 medications are.
GLP-1 medicines act chiefly on appetite centres in the brain: less hunger, earlier fullness, fewer cravings. The resulting drop in calorie intake (about a third at a test meal in one semaglutide study) drives the weight loss. None is described as burning extra calories, and the effect lasts only while the medicine is taken.
What GLP-1 is, and what the medicines do to it
GLP-1 (glucagon-like peptide-1) is released from the gut after eating. The Wegovy SmPC calls it a physiological regulator of appetite and calorie intake, with receptors in several brain areas that regulate appetite. The medicines keep that receptor active far longer than a meal does. Semaglutide (the Wegovy injection and pill) is a modified GLP-1 sharing 94% of the human hormone's sequence; tirzepatide (Mounjaro) activates both the GLP-1 receptor and the receptor for a second gut hormone, GIP; orforglipron (Foundayo) is a small non-peptide molecule absorbed like an ordinary tablet. See our comparison page.
How do GLP-1 drugs cause weight loss? Appetite comes first
The main action is in the brain. The Wegovy SmPC says semaglutide acts directly on the hypothalamus and brainstem, which handle hunger-and-fullness signalling, and on reward-related areas including the amygdala; animal studies show tirzepatide and orforglipron activate nerve cells in the same appetite regions. Measured in people:
- Less eaten. In a phase 1 study, energy intake at a free-choice meal was 35% lower on semaglutide 2.4 mg than on placebo after 20 weeks; tirzepatide also cut intake at such meals.
- Less hunger, more fullness. On tirzepatide, hunger fell from the first week and fullness rose from week 3; semaglutide users reported better control of eating and reduced hunger.
- Changed cravings. Semaglutide reduced cravings for dairy and savoury foods, the desire for sweet food and the preference for high-fat food; tirzepatide reduced cravings for fast-food fats, sweets and starches.
The medicines lower the drive to eat, which is why they are licensed only alongside a reduced-calorie diet and increased physical activity: eating less still produces the loss.
A slower stomach, mostly early on
All four slow stomach emptying, which prolongs fullness and, as the NHS puts it, keeps blood sugar more stable after meals. It is a supporting effect that fades: the Mounjaro and Foundayo SmPCs say the delay diminishes over time (Foundayo's is largest after the first dose), and the Wegovy SmPC describes only a minor early delay. It can also alter how quickly other tablets are absorbed; Foundayo's SmPC warns that oral contraceptives may be less reliable for 30 days after starting and after each dose increase.
Blood sugar and insulin: the diabetes side
The class was developed first for type 2 diabetes. All four prompt insulin release and lower glucagon (the hormone that raises blood sugar) only when blood sugar is high, so on their own they seldom cause hypoglycaemia; the risk rises alongside insulin or a sulfonylurea, which the Mounjaro SmPC flags. Better blood sugar is a benefit, not the cause of the weight loss, which all four SmPCs describe in terms of appetite and food intake.
Why Mounjaro tends to produce more: the GIP question
Only tirzepatide also activates GIP receptors, found on fat cells as well as in the brain. Its SmPC says the weight loss involves decreased food intake and changes in how fat is used, citing animal and laboratory studies of fat cells; how much that adds in people is unsettled. The trial results are clearer, though from separate trials, not head-to-head. Most of the weight lost is fat, but some lean tissue goes too; see GLP-1 medications and muscle loss.
| Medicine | Acts on | Main trial result |
|---|---|---|
| Mounjaro (tirzepatide) | GLP-1 and GIP; weekly injection | 22.5% at 15 mg over 72 weeks (placebo 2.4%), SURMOUNT-1 |
| Wegovy (semaglutide) | GLP-1; weekly injection | 14.9% at 2.4 mg over 68 weeks (placebo 2.4%), STEP 1 |
| Wegovy pill (oral semaglutide) | GLP-1; daily tablet with absorption enhancer | 13.6% at 25 mg over 64 weeks (placebo 2.2%), OASIS 4 |
| Foundayo (orforglipron) | GLP-1; small-molecule daily tablet | 12.4% at 17.2 mg over 72 weeks (placebo 0.9%), ATTAIN-1 |
Why the dose climbs slowly, and the six-month check
Each starts at a low, non-treatment dose and climbs stepwise: Wegovy over about 16 weeks to 2.4 mg, the Wegovy pill over about 12 weeks to 25 mg, Mounjaro in 2.5 mg steps at least four weeks apart to 15 mg, and Foundayo at intervals of at least 30 days to 17.2 mg. The slow climb limits stomach side effects while the appetite effect builds; trial averages were measured at 64 to 72 weeks, so the headline figures reflect over a year of gradual loss. GLP-1 tablets vs injections compares the routines.
A checkpoint is built in for the two injections: their SmPCs and NICE say that if less than 5% of starting weight has been lost after six months (for Mounjaro, six months after reaching the highest tolerated dose), a decision is needed on whether to continue. Not everyone responds.
What happens when you stop
The medicines hold appetite down only while they are in the body. In the STEP 1 extension, people who had lost 17.3% on semaglutide 2.4 mg regained about two-thirds of it in the year after stopping the medicine and the lifestyle programme. In SURMOUNT-4, people switched from tirzepatide to placebo after 36 weeks regained 14.0% of body weight over 52 weeks, while those who continued lost a further 5.5%. The NHS advises talking to your doctor rather than stopping suddenly.
Side effects follow from the mechanism
Nausea, vomiting, diarrhoea and constipation are the most common side effects of all four, typically early on and after dose increases. The Mounjaro SmPC warns that severe vomiting or diarrhoea can cause dehydration that harms kidney function. Less common but serious problems listed include pancreatitis and gallstones. Practical steps are in managing GLP-1 side effects. Report suspected side effects to the MHRA through the Yellow Card scheme; Foundayo carries a black triangle, meaning it is under additional monitoring.
What the mechanism cannot decide
Knowing how GLP-1 drugs cause weight loss does not tell you whether one suits you. A prescriber assesses that (BMI, health conditions, other medicines, pregnancy plans) at a GPhC-registered pharmacy or CQC-regulated clinic, or through the NHS where its criteria are met. See who is eligible for GLP-1 treatment in the UK and the FAQ page. This site names no sellers; private prices vary between regulated providers.
Frequently asked questions
How do GLP-1 drugs cause weight loss in simple terms?
They copy a gut hormone that tells the brain you have eaten. With that signal on all the time you feel less hungry, fill up sooner and crave fewer high-fat and sweet foods, so you take in fewer calories. Eating less is the main cause.
Do GLP-1 drugs speed up your metabolism?
Not according to the UK product information: all four SmPCs describe the weight loss in terms of appetite and food intake; none mentions extra calorie burning. Tirzepatide's extra effect on fat use rests on animal and laboratory studies.
Why does Mounjaro cause more weight loss than Wegovy?
Tirzepatide activates GIP receptors, found on fat cells, as well as GLP-1 receptors. Average loss was 22.5% at 15 mg in SURMOUNT-1 against 14.9% on Wegovy 2.4 mg in STEP 1, but these were separate trials, not a direct comparison.
Do GLP-1 drugs work without changing your diet?
They are licensed only alongside a reduced-calorie diet and increased physical activity, and NICE noted there is no evidence of effectiveness for semaglutide as a stand-alone treatment. The medicine lowers appetite; the weight loss comes from eating less.
Sources
- UK SmPCs (emc): Wegovy FlexTouch, Mounjaro KwikPen, Wegovy 25 mg tablets, Foundayo 17.2 mg tablets.
- NHS Medicines A to Z: Semaglutide, Tirzepatide.
- NICE TA875 (semaglutide) and TA1026 (tirzepatide).
- Wilding JPH et al., STEP 1 extension, Diabetes Obes Metab 2022 (PubMed); Aronne LJ et al., SURMOUNT-4, JAMA 2024 (PubMed).