Short answer: Weight-loss injections are prescription-only injectable medicines that mimic the gut hormones GLP-1 (glucagon-like peptide-1) and GIP. They dampen the appetite centre in the brain, slow down stomach emptying and help balance blood sugar. The main active ingredients are tirzepatide (Mounjaro) and semaglutide (Wegovy, Ozempic).

Key takeaways

  • They mimic the GLP-1 hormone, creating a feeling of fullness in the brain and delaying stomach emptying.
  • In clinical trials (SURMOUNT and STEP), average weight loss of between 15% and 22% of body weight was observed.
  • They are injected once a week under the skin (abdomen, thigh, arm).
  • All weight-loss injections are prescription-only and should be used under medical supervision.

"Weight-loss injection" is the everyday name for medicines known as GLP-1 receptor agonists. These medicines mimic GLP-1, a hormone the gut releases naturally: they prolong the feeling of fullness, slow stomach emptying and reduce appetite. As a result, you eat less and lose weight over time. All of them are prescription-only and need medical follow-up.

This page is for general information only and is not medical advice. Decisions about taking a medicine, the dose and how long to take it belong to your GP or prescriber alone.

How do they work?

GLP-1 is a hormone released by the gut after you eat. It sends an "I'm full" signal to the brain, regulates insulin release from the pancreas and slows the emptying of the stomach. Weight-loss injections mimic the effect of this hormone and produce the same signals for much longer.

One of the newer-generation medicines, tirzepatide (Mounjaro, Zepbound), also acts on the receptor for a second hormone called GIP. In clinical trials, weight-loss results for this dual-acting group were found to be higher than for medicines that act on GLP-1 alone.

Which weight-loss injections are there?

The best-known products and their active ingredients:

Mounjaro

Active ingredient: tirzepatide. Given once a week. Acts on both the GIP and GLP-1 receptors.

Zepbound

Active ingredient: tirzepatide. The same molecule as Mounjaro, sold under a different name for weight management.

Ozempic

Active ingredient: semaglutide. Given once a week. Its primary indication is type 2 diabetes.

Wegovy

Active ingredient: semaglutide. The same molecule as Ozempic, sold for weight management with a different dosing schedule.

Saxenda

Active ingredient: liraglutide. Unlike the others, it is given once a day.

Trulicity

Active ingredient: dulaglutide. Given once a week; its indication is type 2 diabetes.

Two products carrying the same active ingredient (for example Ozempic and Wegovy) come with different dose steps and different indications. Which one is suitable is a medical decision.

To see the differences in active ingredient, dosing frequency and indication for all six products side by side, read weight-loss injection names and comparison table.

How are they used?

These medicines are injected under the skin (subcutaneously). The sites usually used are the abdomen, the front of the thigh and the outer upper arm. Choosing a different site each time is recommended; injecting into the same spot again and again can cause the skin to harden and change how the medicine is absorbed.

Treatment usually starts at the lowest dose and is increased gradually under medical supervision as your body adjusts. This gradual increase is called titration, and it is done to reduce side effects.

What should you watch out for?

  • All of them are prescription-only; they should not be used without a medical assessment.
  • Products sold without a prescription or online may be fake and carry serious risks.
  • The most common side effects are nausea, constipation, diarrhoea and loss of appetite.
  • Keeping them cold matters; storage conditions are given in the patient information leaflet.
  • Enough protein and regular movement are recommended to preserve muscle mass.

Which symptoms are expected, and which mean you should contact a doctor without delay, is covered on a separate page: weight-loss injection side effects and harms.

Tracking your treatment

With a weekly injection routine, the hardest part is remembering what happened last week: which day you injected, which site you used, how your weight changed, which day you felt sick. These are exactly the things your GP or prescriber asks about at a review appointment.

Haftra is a free iPhone app built to keep this record in one place. It logs your injection day and site rotation, your weight trend and your side effects; it can be used without an account, and then your records stay on your phone. It does not suggest doses or make diagnoses; it simply puts your own observations in order.

iPhone, iPad and Apple Watch. Free.

Frequently asked questions

How exactly do weight-loss injections help you lose weight?

They activate the fullness receptors in the hypothalamus area of the brain and lengthen the time it takes for the stomach to empty, so you stay full for longer on smaller portions.

How many times a week is a weight-loss injection given?

Mounjaro, Zepbound, Ozempic and Wegovy are given once a week. Saxenda is used once a day. Your prescriber decides the frequency and the dose.

Who are weight-loss injections prescribed for?

They are usually prescribed by a doctor for people living with obesity who have a body mass index (BMI) of 30 or above, or for people with a BMI above 27 who also have a condition such as high blood pressure or diabetes.

Who are they not suitable for?

They are not used, or need special assessment, in situations such as pregnancy, breastfeeding, a history of certain thyroid cancers or a history of pancreatitis. Whether they are suitable is an assessment only a doctor can make.

Can you get weight-loss injections without a prescription?

No. All of them are prescription-only. Sources selling them without a prescription are not trustworthy; the risk of fake products is real and can have serious consequences.

Does the weight come back when you stop the medicine?

Weight regain is common after stopping: clinical trials show that a large part of the weight lost can be regained once treatment ends. Eating and activity habits built during treatment are still worth keeping, and the decision to stop should be made together with your prescriber. More on this: what happens when you stop.

Detailed pages

Sources

  • NHS: Obesity treatment
  • MHRA (Medicines and Healthcare products Regulatory Agency)

Last reviewed: 2 August 2026