Weight loss medicines explained
Prescription weight-loss medicines such as semaglutide (Wegovy) and tirzepatide (Mounjaro) act on appetite hormones to help people eat less. Most are weekly injections; once-daily tablets are now also available.
In the UK they are licensed for adults with a BMI of 30 or more, or 27 or more with a weight-related health condition such as type 2 diabetes or high blood pressure.
They are prescription-only. A clinician must assess you first and decide whether any medicine is suitable.
The most common side effects are nausea, diarrhoea, constipation and indigestion, usually worst when starting or increasing the dose.
They work alongside changes to diet and activity, not instead of them.
Medically reviewed by Dr Rachel Chander, MBChB, MRCGP, GMC 7149217
CQC Registered (Provider ID: 1-199750102, Location ID 1-565528799)
How do weight loss medicines work
Semaglutide copies a gut hormone called GLP-1. Tirzepatide copies GLP-1 and a second hormone called GIP. Both slow how quickly the stomach empties and act on the brain's appetite centres, so people feel full sooner and for longer. Most are given as a once-weekly injection under the skin; some now come as a once-daily tablet.
More about each medicine:
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Tirzepatide (Mounjaro): how it works, dosing, side effects and safety
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Semaglutide (Wegovy): how it works, dosing, side effects and safety
Medicines
Weekly injections: semaglutide (Wegovy) and tirzepatide (Mounjaro). Daily tablets: oral semaglutide (Wegovy tablets) and orforglipron (Foundayo).
How often
Injections are taken once a week. Tablets are taken once a day.
How to take them
Injections are given under the skin with a pre-filled pen. Oral semaglutide is taken on an empty stomach with a small sip of water, then you wait before eating or drinking. Orforglipron can be taken at any time, with or without food.
Average weight loss in trials
About 15% with semaglutide injections (STEP 1, 68 weeks) and about 21% with the highest dose of tirzepatide (SURMOUNT-1, 72 weeks). About 14% with oral semaglutide 25 mg (OASIS 4, 64 weeks) and about 11% with the highest dose of orforglipron (ATTAIN-1, 72 weeks). These come from separate trials with different people and designs, so they can’t be compared directly, and results vary between individuals.
Common side effects
Similar for injections and tablets; see "What are the side effects?" below.
Trial results come from separate studies and can't be compared directly. Whether any medicine is suitable, and which one, is decided by your clinician after an assessment of your health and other medicines.
Injections or tablets?
Who are they licensed for?
A body mass index (BMI) of 30 kg/m² or greater (obesity) or:
A BMI of at least 27 kg/m² but less than 30 kg/m² (overweight) and weight-related health problems such as:
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Pre-diabetes
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High blood pressure
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Raised cholesterol
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Obstructive sleep apnoea
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Heart or circulation disease
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Type 2 diabetes*
Lower BMI thresholds may apply for some ethnic groups, in line with NICE guidance. They are not suitable for everyone: your clinician checks your medical history, current medicines and goals before deciding.
*People with type 2 diabetes may be able to have weight management at Calleva Clinic, depending on their other diabetes medicines. We do not usually prescribe tirzepatide alongside insulin or a sulfonylurea (such as gliclazide), and we work with your own GP or diabetes team to keep your care joined up. Our assessments are for adults aged 18 to 85 living in England.
How much weight do people lose?
Average weight loss in the main trials:
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Semaglutide injection: about 15% over 68 weeks (STEP 1)
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Tirzepatide injection, highest dose: about 21% over 72 weeks (SURMOUNT-1)
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Oral semaglutide 25 mg: about 14% over 64 weeks (OASIS 4)
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Orforglipron, highest dose: about 11% over 72 weeks (ATTAIN-1)
These are separate trials, so they can't be compared directly. Results vary between people, and weight often returns if treatment stops.
What are the side effects?
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Common: nausea, vomiting, diarrhoea, constipation, indigestion, tiredness. These usually ease as the body adjusts.
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Less common but serious: pancreatitis (severe stomach pain spreading to the back), gallbladder problems, and dehydration that can affect the kidneys. Seek urgent medical help for severe or persistent stomach pain.
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Low blood sugar: possible if you also take insulin or a sulfonylurea for diabetes.
Pregnancy and contraception
These medicines must not be used in pregnancy or while breastfeeding. The MHRA advises:
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Use effective contraception while taking them.
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With tirzepatide, use a non-oral method (such as a condom or coil) for 4 weeks after starting and for 4 weeks after each dose increase, because it can make the pill less effective.
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Stop semaglutide at least 2 months, and tirzepatide at least 1 month, before trying to get pregnant.
Who should not take them?
Your clinician will discuss this with you. They are usually not suitable if you are pregnant, planning pregnancy or breastfeeding, have had pancreatitis, or have certain other conditions. Always tell the clinician about every medicine you take.
Keeping muscle
WHILE LOSING WEIGHT
Rapid weight loss can include muscle as well as fat. Eating enough protein and doing regular strength exercise helps. Read our guide to preserving muscle.
How does a consultation work
AT CALLEVA CLINIC
You book a free video consultation with our GP-led team. The clinician reviews your health and goals. If a prescription medicine is suitable, they explain the options, risks and costs; if not, they recommend other support. Our prices are listed on the treatment fee schedule.
Frequently Asked Questions
Sources: MHRA Drug Safety Update on GLP-1 medicines and contraception (June 2025); Wegovy and Mounjaro SPCs on the electronic medicines compendium; NICE TA875 (semaglutide) and TA1026 (tirzepatide); the STEP 1 and SURMOUNT-1 trial papers.
Contact
Calleva Clinic
Online GP-led clinic: video consultations across England
Phone: 0118 214 1879
Your call is first answered by Ava, our AI receptionist. Anything that needs our team is passed on, and we’ll follow up by email or phone.