What are weight loss injections?
Weight loss injections are medicines that you inject under the skin once a week. The two most widely discussed in the UK are Mounjaro, which contains tirzepatide, and Wegovy, which contains semaglutide. Both are licensed for weight management as an add-on to a reduced-calorie diet and more physical activity.
Both active ingredients were first licensed for type 2 diabetes, under other brand names, before weight management licences were granted. Check the product name on your prescription, as the same ingredient can appear under different names.
Under the licence, adults can be treated if their starting BMI is 30 or above, or 27 to under 30 with at least one weight-related condition such as high blood pressure, dyslipidaemia, obstructive sleep apnoea or type 2 diabetes. The summary of product characteristics (SmPC) for each medicine sets out these criteria.
How the GLP-1 hormone works in the body
GLP-1 stands for glucagon-like peptide-1. Your gut releases it after you eat. It is one of the hormones that tell your body that a meal has arrived, and it helps regulate blood sugar.
Wegovy is a GLP-1 receptor agonist. Semaglutide attaches to the same receptors as natural GLP-1 and keeps them switched on for longer, because the medicine breaks down far more slowly than the hormone itself. In general terms, the signals this produces include:
- reduced appetite and a sense of fullness after smaller amounts of food
- slower emptying of the stomach, so food stays in the stomach for longer
- insulin release that rises when blood sugar is high
The appetite effect is the one most people notice. Because food stays in the stomach for longer, nausea and feeling bloated are common early on. That is one reason the dose is increased slowly.
What does the GIP part of Mounjaro do?
Mounjaro contains tirzepatide, which is a long-acting GIP and GLP-1 receptor agonist. GIP, or glucose-dependent insulinotropic polypeptide, is another gut hormone that is involved in insulin release and fat handling. According to the Mounjaro SmPC, tirzepatide binds strongly to both the GIP and GLP-1 receptors.
Because the two hormone actions are combined in one medicine, Mounjaro is sometimes called a dual agonist. Researchers are still working out how much of the weight loss comes from each action. What matters for most people is the overall result in trials, and the side effect profile, which is covered in our safety guide.
Wegovy acts on the GLP-1 receptor only. This is one of the differences you will see when you compare the two medicines in Mounjaro vs Wegovy.
How much weight do people lose on weight loss injections?
Trial averages are the best guide to what these medicines do across a group of people. They are not a promise for any one person. In both trials below, everyone also received lifestyle support.
| Trial | Medicine | Length | Average weight change on medicine | Average change on placebo |
|---|---|---|---|---|
| STEP-1 (Wilding et al., NEJM 2021) | Semaglutide 2.4 mg | 68 weeks | -14.9% | -2.4% |
| SURMOUNT-1 (Jastreboff et al., NEJM 2022) | Tirzepatide 15 mg | 72 weeks | -20.9% | -3.1% |
STEP-1 included adults with obesity or overweight who did not have diabetes. SURMOUNT-1 included adults with obesity who did not have diabetes. The two trials used different populations and designs, so the numbers should not be read as a direct head-to-head comparison.
Weight loss in the trials was also not the same for everyone. Some participants lost much less than the average, and some stopped early because of side effects. The head-to-head comparison in SURMOUNT-5 is covered in Mounjaro vs Wegovy.
Why does the dose start low and increase slowly?
Both medicines are started at a low dose and increased in steps every few weeks. The aim is to let your stomach and bowel adjust. A slow increase gives your gut time to adjust, which can make side effects easier to manage.
The exact steps are different for each medicine. Mounjaro starts at 2.5 mg once a week for 4 weeks and rises in 2.5 mg steps, with at least 4 weeks at each dose. Wegovy starts at 0.25 mg and reaches 2.4 mg after about 16 weeks. Full dose tables are in our dose schedule guide.
Do not change your dose on your own. If you are not able to tolerate a step, speak to your prescriber, who can decide whether to pause, reduce the dose or stop.
Can you get weight loss injections on the NHS in Northern Ireland?
At the time of writing, routine NHS prescribing of Mounjaro or Wegovy for weight loss is not available in Northern Ireland. The Northern Ireland formulary entry for tirzepatide, dated April 2025, states that it is not currently available through the health service for obesity.
On 29 June 2026, the Department announced the first phase of the Regional Obesity Management Service (ROMS), a £5 million specialist service. Phase one is for adults aged 18 and over with a BMI above 45, plus at least one of several listed conditions, including type 2 diabetes and obstructive sleep apnoea confirmed by a sleep study. The Department says the service will provide weight-loss medication "if clinically appropriate and in line with NICE guidance". Rollout is expected in early autumn 2026.
A separate programme, the Obesity Pathway Innovation Programme, is aimed at people with a BMI of 30 to 39.9, with delivery starting in August 2026. Eligibility and timings can change, so check the Department of Health website before you act on them. Our NHS access guide for Northern Ireland explains the routes in more detail, and the eligibility checker gives a general indication of where you may stand.
Private prescriptions are available from registered providers. Read the buying guide before ordering, and check that any pharmacy is registered with the Pharmaceutical Society of Northern Ireland (PSNI).
What happens if you stop weight loss injections?
These medicines help people lose weight while they are taken. They are not a cure for obesity, and weight tends to return when treatment stops. In SURMOUNT-4, adults who had lost weight on tirzepatide and then switched to placebo regained weight over the following year, while those who stayed on tirzepatide kept most of their loss. This was a secondary summary of the trial, so check the published paper if you need exact figures.
Many people are advised to keep up the healthy eating and activity habits they built during treatment. Our guide on stopping weight loss injections covers what to plan for before a decision is made.
Whether these medicines are right for you depends on your health, your weight history and what else you take. Speak to your GP or prescriber, or to a pharmacist if you have questions about a specific medicine.