GLP-1 in Plain English: What It Does, Why It Works and What’s New

In 30 seconds

• GLP-1 is a natural gut hormone released after you eat.
• It helps the body respond to food by influencing insulin, appetite, fullness and digestion.
• GLP-1 medicines make this signal stronger or last much longer than natural GLP-1.
• Some are used for type 2 diabetes, some for weight management, and the licensed purpose depends on the medicine.
• Mounjaro is slightly different: tirzepatide acts on both GLP-1 and another hormone system called GIP.
• These medicines can produce substantial weight loss in some people, but they are not risk-free and are not suitable for everyone.
• Weight can return after treatment stops, so the long-term plan matters as much as the initial weight loss.

Why is everyone suddenly talking about GLP-1?

For years, GLP-1 medicines were mainly discussed in diabetes clinics.

That changed when newer medicines produced levels of weight loss that were much greater than people had come to expect from older weight-management medicines.

In the major STEP 1 trial, adults with overweight or obesity who received weekly semaglutide 2.4 mg alongside lifestyle intervention lost an average of about 14.9% of their starting body weight over 68 weeks, compared with 2.4% with placebo.

In the separate SURMOUNT-1 trial, people receiving the highest tested dose of tirzepatide lost an average of about 20.9% over 72 weeks, compared with 3.1% with placebo.

But they help explain the excitement. These medicines can have a major biological effect on hunger and food intake.

What is GLP-1?

GLP-1 stands for glucagon-like peptide-1.

It is one of the hormones your gut releases when food arrives.

Think of it as part of the conversation between your digestive system, pancreas and brain.

To the pancreas

When blood glucose rises, GLP-1 helps the pancreas release insulin.

Insulin helps move glucose out of the bloodstream and into tissues where it can be used or stored.

To the brain

GLP-1 signalling can reduce hunger and increase feelings of fullness.

Some people taking GLP-1 medicines describe having less persistent thinking about food — sometimes informally called “food noise.”

To the digestive system

GLP-1 can slow how quickly food leaves the stomach.

That can help someone feel full for longer after eating.

So do GLP-1 medicines simply “burn fat”?

No.

That description is misleading.

A major reason they help with weight loss is that many people feel less hungry, become full sooner and therefore eat less.

Over time, the resulting reduction in energy intake can lead to substantial weight loss.

The medicines also have important effects on blood-glucose regulation.

Ozempic, Wegovy and Mounjaro: are they all the same thing?

No — and the brand names can make this unnecessarily confusing.

Semaglutide

Semaglutide is the active medicine used in several products.

• Ozempic is used for type 2 diabetes.
• Rybelsus is an oral semaglutide product used for type 2 diabetes.
• Wegovy is semaglutide licensed for weight management and also has a cardiovascular-risk-reduction indication for certain people.

The important point is that the same active drug can be sold under different brand names for different licensed purposes.

Tirzepatide

Mounjaro contains tirzepatide.

It is often included in everyday conversations about “GLP-1 drugs”, but technically it acts on two hormone receptors:

• GLP-1
• GIP — glucose-dependent insulinotropic polypeptide

It is therefore more accurately described as a dual GIP/GLP-1 receptor agonist.

What else can these medicines do besides reduce weight?

It depends on the medicine and the person.

GLP-1-based medicines were originally developed around blood-glucose control, and several are established treatments for type 2 diabetes.

Evidence has since expanded.

A cardiovascular example

A particularly important example is the SELECT trial.

More than 17,000 adults with established cardiovascular disease and overweight or obesity — but without diabetes — were studied.

Major cardiovascular events occurred in 6.5% of people receiving semaglutide compared with 8.0% receiving placebo during the trial.

In May 2026, NICE recommended semaglutide as an NHS option for reducing major cardiovascular events in defined adults with established cardiovascular disease and a BMI of at least 27 kg/m².

What happens when you stop?

This is one of the most important questions — and one of the easiest to overlook when attention is focused on rapid weight loss.

In an extension of the STEP 1 trial, people who had stopped semaglutide regained, on average, around two-thirds of the weight they had previously lost during the following year.

That does not mean everyone will regain exactly the same amount.

It does tell us something important:

For many people, treatment therefore needs to be thought about as part of a longer-term strategy rather than a short course followed by “problem solved”.

Before starting, it is sensible to ask: What is my maintenance plan if this medicine works?

Eating less means food quality matters more

Reduced appetite can make eating less much easier.

But eating less creates another question:

Someone consuming much smaller portions still needs adequate:

• Protein
• Fibre
• Vitamins and minerals
• Essential fats
• Fluids
• Overall nutritional variety

This can be especially relevant for older adults and anyone who wants to preserve strength and muscle while losing weight.

Does GLP-1 treatment cause muscle loss?

Weight loss usually includes some loss of lean tissue as well as fat.

In an exploratory body-composition analysis from STEP 1, semaglutide produced much larger reductions in fat mass than lean mass, but absolute lean body mass also decreased.

That does not mean GLP-1 medicines uniquely “destroy muscle”.

It means that when substantial weight is being lost, preserving muscle deserves attention.

Appropriate resistance exercise, good dietary quality and adequate protein may therefore be important parts of a wider weight-management plan. The right approach will vary with age, kidney function, health conditions, activity and overall diet.

Can I increase GLP-1 naturally instead?

Your body already releases GLP-1 after food.

Different nutrients and eating patterns can influence normal gut-hormone responses.

But this should not be confused with taking a medicine specifically designed to stimulate the GLP-1 receptor strongly and for a prolonged period.

A healthy diet can support appetite control and metabolic health. That does not make a food equivalent to semaglutide or another prescription medicine.

What are the common side effects?

The most frequent side effects are digestive.

They can include:

• Nausea
• Vomiting
• Diarrhoea
• Constipation
• Abdominal discomfort

They are often most noticeable when treatment begins or the dose increases.

For many people they improve with time, but not always.

Vomiting or diarrhoea can also lead to dehydration.

What are the more serious risks?

Serious reactions are much less common, but they matter.

Pancreatitis

In January 2026, the UK medicines regulator strengthened warnings about acute pancreatitis with GLP-1 and dual GIP/GLP-1 medicines, including rare reports of severe, necrotising and fatal cases.

Sudden changes in vision

In February 2026, UK product information for semaglutide was updated because of a very rare possible association with a condition called non-arteritic anterior ischaemic optic neuropathy — NAION.

It can cause sudden, painless loss or deterioration of vision, often in one eye.

A sudden loss of vision while taking semaglutide needs urgent medical assessment.

Pregnancy

GLP-1 medicines should not be used during pregnancy.

Anyone who may become pregnant should discuss contraception, stopping periods before pregnancy and their particular medicine with an appropriate healthcare professional.

Tirzepatide can also affect the reliability of oral contraception around treatment initiation and dose increases, so specific advice may be needed.

Surgery or sedation

Because these medicines can slow stomach emptying, your anaesthetist or surgical team needs to know if you use one.

Do not stop prescribed medicine on your own. Follow the advice of the clinical team responsible for your procedure.

What we know — and what we should not assume

What we know

GLP-1-based medicines can:

• Improve blood-glucose control in appropriate people
• Substantially reduce body weight in many people with obesity
• Reduce appetite and increase fullness
• Produce important cardiovascular benefits with certain medicines in certain high-risk populations

What we should not assume

We should not assume that:

• Every GLP-1 medicine is identical
• Everyone will lose the same amount of weight
• Every person with excess weight needs medication
• Every claimed benefit belongs to the whole drug class
• Weight loss will automatically remain after treatment stops
• A medicine removes the need for good nutrition and physical activity
• Something is safe simply because it is described online as a “weight-loss jab”

What is new right now? — August 2026

This is one reason GLP-1 deserves to be a living page.

The story is still moving quickly.

We will update this page as the evidence, safety guidance and access picture develops.

Would testing tell me whether I need a GLP-1?

There is no routine blood test where someone measures your GLP-1 level and then decides that you “need more GLP-1”.

That is not how these medicines are prescribed.

Assessment depends on the reason treatment is being considered.

Relevant information may include:

• BMI and weight history
• Type 2 diabetes or blood-glucose status
• Cardiovascular disease
• Other weight-related health conditions
• Current medicines
• Previous medical history
• Pregnancy plans
• Previous attempts at weight management
• Appropriate blood tests and clinical assessment where required

The exact assessment should be decided by the healthcare professional responsible for prescribing.

Where does HTMA fit?

It cannot diagnose diabetes, establish whether semaglutide or tirzepatide is suitable, monitor drug safety or replace the clinical assessment required for prescription treatment.

For this reason, HTMA should not be positioned as a GLP-1 test.

A broader nutritional assessment may sometimes be useful for someone whose appetite and food intake have changed substantially, but that is a different question.

What can you do now?

1. Understand what you are actually considering.
   Do not use “Ozempic”, “GLP-1” and “weight-loss injection” as if they all mean exactly the same thing. Find out the active drug, brand and licensed purpose.
2. Use a legitimate prescriber and pharmacy.
   Do not buy injectable or tablet weight-loss medicines from unregulated sellers, social media accounts or websites that bypass proper prescribing.
3. Think beyond the scales.
   If appetite becomes much smaller, consider the nutritional quality of what remains. Protein, fibre, fluids, vitamins, minerals and overall dietary variety still matter.
4. Protect the long-term result.
   Ask what happens after the initial weight loss. A maintenance strategy should not begin on the day treatment stops.
5. Know the warning signs.
   Persistent severe abdominal pain, significant dehydration or sudden changes in vision require appropriate medical attention.

How we can help

GLP-1 medicines are prescription treatments.

Our nutrition consultations do not replace the healthcare professional prescribing or monitoring them.

Where we can help is with the nutritional side of the picture.

The aim is not to compete with medical treatment. It is to make sure the nutrition surrounding that treatment is not forgotten.

Research record

First published: 21 August 2026
Last reviewed: 21 August 2026
Latest evidence update: 21 August 2026

Key evidence and guidance

• Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021. PMID 33567185.
• Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022. PMID 35658024.
• Wilding JPH, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: The STEP 1 trial extension. Diabetes Obes Metab. 2022. PMID 35441470.
• Lincoff AM, et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. N Engl J Med. 2023. PMID 37952131.
• NICE. Semaglutide for reducing the risk of major adverse cardiovascular events in people with cardiovascular disease and overweight or obesity. Technology appraisal TA1152. Published 7 May 2026.
• Medicines and Healthcare products Regulatory Agency. First GLP-1 tablet for weight loss approved in the UK. 11 June 2026.
• Medicines and Healthcare products Regulatory Agency. UK first in Europe to authorise orforglipron for weight management and type 2 diabetes. 10 August 2026.
• Medicines and Healthcare products Regulatory Agency. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases. 29 January 2026.
• Medicines and Healthcare products Regulatory Agency. Semaglutide: risk of non-arteritic anterior ischaemic optic neuropathy (NAION). 5 February 2026.