Weight loss injections: How do weight loss drugs like Mounjaro work and are there long-term side effects?

• Weight loss injections are becoming a popular option for those wanting to lose weight.
• Drugs like Mounjaro and Wegovy are now household names.
• NICE has approved Mounjaro for weight loss treatment for those who meet the criteria in the UK.

Weight loss injections including Mounjaro and Wegovy have became household names. The medications, which were originally a treatment for type 2 diabetes, have been helping those living with obesity lose weight.

The 2022 Health Survey for England estimated that 28% of adults were obese and 36% were classified as being overweight.

Medications to help lose weight have been gaining in popularity, with NHS England approving prescriptions for Tirzepatide (also called Mounjaro), from March 2025 to those who meet the criteria.

Speaking in October 2024 about Mounjaro being made available on the NHS, Dr Sam Roberts, chief executive of NICE said: “This drug will be a powerful part of our arsenal to tackle obesity and support many more people to lose weight and reduce their risk of diabetes, heart attack and stroke.

“This phased rollout will ensure those with the greatest clinical need can access it as a priority – with a quarter of a million people able to benefit over the first three years – while we develop new and innovative services through which other weight loss treatments can also be delivered.” 

So, what do we know about weight loss injections, how do they work and are there any long-term side effects? Here’s everything you need to know.

How do weight loss injections work?

Weight loss injections work by regulating your appetite by mimicking a naturally produced hormone called glucagon-like peptide-1 (GLP-1). Superdrug explain that normally when we eat food, GLP-1 is released, so the medication makes your brain feel you are full and reduces feelings of hunger.

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Are there any side effects?

Weight loss injections may cause side effects for some people. Boots explain that they should be mild and improve or even disappear after a few weeks of starting your treatment.

Side effects can include: nausea or heartburn, constipation, headache, vomiting, diarrhoea, injection site reactions, gallstones and inflammation of the gall bladder.

If you are taking Mounjaro or Wegovy, all the side effects that you experience should be reported using the Yellow Card Scheme.

What are the long term side effects?

Weight loss injections are long-term treatments, with prescriptions for Wegovy available for two years.

According to the British Heart Foundation some research suggests that they can help reduce diseases including heart attacks and strokes.

However, rare but serious side-effects could include gall bladder disease, inflammation of the pancreas and kidney issues.

The long-term side effects of weight loss injections are still being studied, there is still a lot that is not known about how taking GLP-1 agonists can affect different areas of the body.

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