How weight loss injections work
Mounjaro (active ingredient tirzepatide) and Wegovy (active ingredient semaglutide) are prescription-only injectable medicines. They mimic natural hormones released by the gut after eating, particularly GLP-1 (glucagon-like peptide-1). Mounjaro also acts on a second hormone, GIP (glucose-dependent insulinotropic polypeptide).
These hormones are involved in:
- Appetite: they signal fullness to the brain, which can reduce how much you want to eat.
- Stomach emptying: they slow the rate food leaves the stomach, so the feeling of fullness after a meal lasts longer.
- Blood sugar: they help regulate blood sugar and how the body handles food.
Used under the guidance of a healthcare professional, alongside healthier eating and more activity, they can help people lose a meaningful amount of weight and keep it off.
Mounjaro (tirzepatide)
Mounjaro is a once-weekly injection that acts on both GLP-1 and GIP. In the SURMOUNT-1 clinical trial, adults without diabetes taking the highest dose lost about 21% of their body weight on average over 72 weeks, alongside diet and activity changes.
Treatment starts at 2.5mg once a week for four weeks. Your prescriber may then increase the dose in 2.5mg steps, at least four weeks apart, to find the dose that works for you, between 5mg and 15mg.
Mounjaro prices and details · The 2026 Mounjaro KwikPen update
Wegovy (semaglutide)
Wegovy is a once-weekly injection that acts on GLP-1. In the STEP 1 clinical trial, adults taking the 2.4mg dose lost about 15% of their body weight on average over 68 weeks, alongside diet and activity changes. Wegovy has been reviewed by NICE for weight management.
Treatment starts at 0.25mg once a week and increases every four weeks, usually reaching the 2.4mg maintenance dose after 16 weeks. Some people stay on 1.7mg.
Wegovy is also available as a once-daily tablet. Read our guide to switching to Wegovy tablets.
Who might consider weight loss injections?
Weight loss injections are for adults who meet specific Body Mass Index (BMI) criteria:
- a BMI of 30 or more, or
- a BMI of 27 or more with a weight-related condition, such as high blood pressure, high cholesterol, pre-diabetes, sleep apnoea or heart disease.
They may be an option if you have found it hard to lose weight with diet and exercise alone, are ready to make long-term lifestyle changes, and are happy to have regular check-ins with a prescriber.
An online service isn't the safest route if you: have diabetes; are pregnant, planning a pregnancy or breastfeeding; have had an eating disorder, pancreatitis or severe stomach-emptying problems; or have a personal or family history of medullary thyroid cancer or MEN2. If any of these apply, please speak to your GP.
How our consultation process works
Our service is pharmacist and nurse-led and fully online:
- Free online consultation. Answer questions about your weight, health history, medicines and goals. It takes about five minutes.
- Video consultation. Before any first prescription, you meet a pharmacist or nurse prescriber on a video call. They check your details, talk through your options and side effects, and show you how to use an injection pen if that's what you choose.
- Delivery and ongoing support. If treatment is right for you, it's dispensed by our registered pharmacy and delivered free. Your prescriber reviews your progress and side effects before each dose change.
Side effects and safety
As with all medicines, weight loss injections can cause side effects. The most common are feeling sick, diarrhoea, vomiting, constipation and tummy discomfort. These are usually mild to moderate and tend to settle as your body adjusts, especially if dose increases are gradual.
Get urgent medical help if you have severe or persistent stomach pain (which can spread to your back and may be a sign of pancreatitis), a severe allergic reaction, or vomiting or diarrhoea that leaves you dehydrated.
Rarer but more serious side effects are discussed during your consultation. You can report suspected side effects through the MHRA Yellow Card scheme.
The role of lifestyle
Weight loss injections are designed to support your own efforts and work best as part of a broader plan. Lasting results usually come from combining treatment with consistent healthy eating, regular physical activity and, where helpful, behavioural support.
Your prescriber will help you set realistic changes that fit your life, and plan with you what happens when you reach your goal, whether that's continuing, a maintenance dose, or stopping safely.
Talk it through with a prescriber
Start with our free online consultation. Before any treatment starts, you'll have a video consultation with a pharmacist or nurse prescriber, who will recommend what's safest for you.
Start free consultationFrequently asked questions
How do weight loss injections work?
They mimic natural gut hormones such as GLP-1 that signal fullness to the brain and slow stomach emptying, so you feel full sooner and for longer. Mounjaro also acts on a second hormone, GIP.
What is the difference between Mounjaro and Wegovy?
Mounjaro contains tirzepatide, which acts on GLP-1 and GIP. Wegovy contains semaglutide, which acts on GLP-1. Both are once-weekly injections. Your prescriber will recommend what suits your health and goals.
Who can have weight loss injections?
Adults with a BMI of 30 or more, or 27 or more with a weight-related condition, following an assessment by a prescriber. Some conditions mean an online service isn't suitable.
Do I need a consultation?
Yes. They are prescription-only medicines. Our online consultation is free, and every first prescription starts with a video consultation with a pharmacist or nurse prescriber.
Do I still need to diet and exercise?
Yes. The medicines are licensed for use alongside a reduced-calorie diet and increased physical activity.
How are the pens delivered?
Free tracked delivery in plain packaging, with cool packaging for pens. Refrigerate them as soon as they arrive.
References
- NHS: Obesity treatment
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). NEJM 2022
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). NEJM 2021
- NICE TA875: Semaglutide for managing overweight and obesity
- NICE TA1026: Tirzepatide for managing overweight and obesity
- MHRA Yellow Card scheme
Last reviewed: October 2026. This guide is general information and doesn't replace advice from your prescriber or the leaflet supplied with your medicine.