How weight loss injections and pills work
Weight loss injections and pills copy gut hormones which tell your brain you have eaten enough. This guide explains GLP-1 and GIP, the difference between pens and tablets, and the reason every dose rises in steps.
In short: All four medicines switch on the GLP-1 receptor, the target of a hormone your gut releases after eating. Mounjaro also switches on the GIP receptor. The result is less hunger, earlier fullness and fewer cravings. Mounjaro and Wegovy are weekly injections with half-lives of about 5 days and about 1 week. The Wegovy pill and Foundayo are daily tablets. Appetite returns once the medicine clears.
GLP-1 and GIP in plain language #
GLP-1 is a hormone your gut releases after you eat. The Wegovy product document describes GLP-1 as a natural regulator of appetite and calorie intake. GIP is a second gut hormone. GIP receptors sit on the pancreas, on fat cells and in brain areas controlling appetite.
A medicine which switches on a hormone's receptor is called an agonist. Semaglutide (Wegovy) and orforglipron (Foundayo) are GLP-1 receptor agonists. Tirzepatide (Mounjaro) acts on both the GIP and GLP-1 receptors. More background sits in what is GLP-1.
What happens in the brain, gut and pancreas #
Brain
Semaglutide acts on the hypothalamus and brainstem, the areas controlling hunger and food intake, and on areas linked to the reward value of food. People report more fullness, less hunger, fewer cravings and a lower preference for high-fat food. In one study, people on semaglutide 2.4 mg for 20 weeks ate 35% less energy at a free-choice meal than people on placebo. With tirzepatide, hunger scores fell from week 1 and fullness rose from week 3.
Gut
The medicines slow stomach emptying, so food stays in your stomach longer after a meal. Semaglutide causes a minor delay in the early period after eating. With tirzepatide the delay lessens over time. Slow emptying is the reason anaesthetists need to know you take one of these medicines.
Pancreas
When blood sugar is high, the medicines raise insulin release and lower glucagon, a hormone which pushes blood sugar up. The effect depends on the glucose level. When blood sugar is low, semaglutide reduces insulin release. Low blood sugar is mainly a concern for people who also take insulin or a sulfonylurea. See GLP-1 medicines and diabetes.
The four medicines side by side #
| Medicine | Target | Form | How often | Half-life |
|---|---|---|---|---|
| Mounjaro (tirzepatide) | GIP and GLP-1 receptors | Peptide in an injection pen | Once a week | About 5 days |
| Wegovy (semaglutide) | GLP-1 receptor | Peptide in an injection pen | Once a week | About 1 week |
| Wegovy pill (semaglutide) | GLP-1 receptor | Peptide tablet with an absorption enhancer | Once a day on an empty stomach | About 1 week |
| Foundayo (orforglipron) | GLP-1 receptor | Non-peptide small-molecule tablet | Once a day at any time | 29 to 49 hours |
Read the full guides to Mounjaro, Wegovy, the Wegovy pill and Foundayo.
Peptide injections, peptide tablets and small-molecule tablets #
Tirzepatide and semaglutide are peptides, short chains of amino acids. Semaglutide shares 94% of the structure of human GLP-1. Both launched as injections under the skin.
The Wegovy pill carries semaglutide through the stomach wall with a helper ingredient called salcaprozate sodium (SNAC). Even so, only about 1% to 2% of each tablet reaches your blood. The top tablet dose is 25 mg a day for this reason, while the standard weekly injection dose is 2.4 mg. Food and large drinks cut absorption further, so the rules are strict. Take the tablet after at least 8 hours without food, with up to 120 ml of water, then wait at least 30 minutes before eating, drinking or taking other oral medicines.
Foundayo is a different kind of medicine. Orforglipron is a non-peptide small molecule. You take one tablet whole at any time of day, with no food or water rules.
Why doses rise in steps #
Each of the four medicines starts low. The Wegovy product document gives the reason: to reduce the likelihood of stomach and bowel symptoms. Nausea, vomiting and diarrhoea are most frequent during dose increases and ease over time.
- Mounjaro: 2.5 mg weekly for 4 weeks, then 5 mg, then rises of 2.5 mg after at least 4 weeks on each dose, up to 15 mg.
- Wegovy injection: 0.25 mg, 0.5 mg, 1 mg and 1.7 mg for 4 weeks each, reaching 2.4 mg after 16 weeks. A 7.2 mg dose is an option after at least 4 weeks on 2.4 mg.
- Wegovy pill: 1.5 mg, 4 mg, 9 mg, then 25 mg, with at least 1 month on each.
- Foundayo: 0.8 mg, 2.5 mg, 5.5 mg, then 9 mg, 14.5 mg or 17.2 mg, with at least 30 days on each.
Long half-lives fit the monthly pattern. Semaglutide takes 4 to 5 weeks at one dose to reach a steady level in your blood. If side effects are strong, the prescriber can delay the next step or go back one dose.
Why appetite returns after stopping #
The medicines work only while present in your body. After a last 2.4 mg Wegovy injection, semaglutide stays in the circulation for about 7 weeks. Orforglipron clears in roughly 6 to 10 days. Once the medicine has gone, the extra GLP-1 signal stops.
Trials show the result. In SURMOUNT-4, adults took tirzepatide 10 mg or 15 mg weekly for 36 weeks with lifestyle counselling and lost 20.9% of body weight. Those then switched to placebo regained 14.0% over 52 weeks, while those who stayed on tirzepatide lost a further 5.5%. After STEP 1 (semaglutide 2.4 mg weekly for 68 weeks with diet and activity support), participants followed for a year off treatment ended 5.6% below starting weight.
The medicines do not reset appetite permanently. Habits built during treatment and a planned step-down both matter. Read about maintenance doses and preventing weight regain.
How weight loss injections and pills work: questions #
What is the difference between GLP-1 and GIP?
Both are hormones your gut releases after eating. GLP-1 reduces appetite, slows stomach emptying and raises insulin when blood sugar is high. GIP receptors sit on the pancreas, on fat cells and in appetite areas of the brain. Wegovy and Foundayo act on the GLP-1 receptor only. Mounjaro acts on both receptors.
Why is the Wegovy pill dose higher than the injection dose?
Tablets and injections deliver different proportions of semaglutide into the bloodstream. Oral absorption is low even with the absorption helper SNAC, so the tablet uses a different dose schedule. Do not convert milligrams between presentations or switch without prescribing instructions.
How long do weight loss injections stay in your system?
These medicines leave the body gradually over several weeks. Semaglutide’s half-life is about one week; tirzepatide’s is about five days. Pregnancy planning requires separate medicine-specific stopping advice from your prescriber, rather than estimating safety from the date of the last injection.
Do weight loss pills work as well as injections?
Results from separate trials cannot reliably rank pills against injections. Studies use different medicines, doses, participants, follow-up periods and analyses. Discuss the evidence for each option with your prescriber; average trial results do not predict your individual outcome.
Why do I have to start on a low dose?
Gradual dose escalation helps reduce gastrointestinal side effects. Each medicine has its own licensed starting dose and schedule. Follow your prescription, report troublesome symptoms and ask your prescriber before moving to a higher dose or changing the timing.
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Page information
- Summary
- How GLP-1 and GIP medicines cut appetite, why doses rise in steps, how tablets differ from pens and why hunger returns after you stop. Read the full guide.
- Publisher
- Aero Clinic, operated by AERO HEALTHCARE LTD.
- Editorial update
- Clinical review
- Pending. An editorial update is not a clinical sign-off.
Cite as: How weight loss injections and pills work, Aero Clinic (https://www.cagri.co.uk), AERO HEALTHCARE LTD. https://www.cagri.co.uk/how-weight-loss-injections-work. Editorial update: 4 November 2026. Clinical review pending.