As Wegovy/Ozempic becomes available in New Zealand for the first time, the Telegraph’s David Cox explains how it – and other so-called GLP-1 class drugs – work.

When it comes to weight loss, we’re all after a magic bullet, a simple, stress-free way of shedding the excess pounds. And so for millions around the world, the arrival of the so-called GLP-1 class of weight-loss drugs in 2021 seemed like a godsend, especially for anyone who’s struggled to stick with dieting advice, commit to an exercise regime or simply find a weight-loss solution that works.

So with that in mind, how exactly do GLP-1 drugs work, what’s out there, and who is eligible to get them?

What are GLP-1s and how did it all start?

The story of a drug market, and the origins of GLP-1, now worth hundreds of billions, began with a surprise discovery at Massachusetts General Hospital in the 1980s.

Svetlana Mojsov, a biochemist at The Rockefeller University, carried out a series of experiments which revealed that the gut releases a hormone called GLP-1 in response to eating, stimulating the release of insulin in the pancreas. Further research would later show that GLP-1 also acts on the brain to signal that we’re satiated and dampen down appetite.

Over more than two decades, scientists at Danish pharmaceutical company Novo Nordisk and American pharma Eli Lilly would work to develop synthetic versions of this natural hormone; injectable medications called GLP-1 receptor agonists, or more commonly, GLP-1s. The idea was that they would bind to and activate the same biological pathways in the body as the real thing, but in a vastly more potent fashion which lasted for days at a time.

GLP-1s were initially marketed to Type 2 diabetes patients because of their ability to stimulate the production of insulin when blood sugar levels are excessively high. The more recent suite of medications are administered in higher doses, with the primary goal of facilitating weight loss.

“The drugs that work on this system are much more powerful than the natural GLP-1 hormone,” explains Professor John Wilding, an expert in obesity, diabetes and endocrinology at the University of Liverpool. “The natural hormone is broken down within a few minutes, whereas you inject the drugs and they last in the body for a week or longer.”

How do GLP-1 drugs work?

Scientists have discovered that both the vagus nerve – a vast chain of nerve fibres which spans the body – and the brain itself are studded with so-called GLP-1 receptors. These are proteins which can be specifically stimulated by either the body’s own GLP-1 hormones produced by the gut, or synthetic GLP-1 drugs such as Ozempic and Mounjaro.

Wilding says that the presence of GLP-1 receptors in the brain is thought to be one of the key reasons why GLP-1 drugs can impact hunger, fullness and reward, as they allow these drugs to stimulate various brain regions, swiftly dampening down appetite and the desire to overeat.

But beyond weight loss, there also may be some unforeseen, even more profound, consequences of stimulating GLP-1 receptors in the brain, which is why GLP-1 drugs are also being studied for conditions ranging from addiction to neurodegenerative diseases. Because these receptors are not only found in the brain, but across the body, including in organs such as the kidney and the heart, they may also have beneficial consequences when it comes to preventing and even treating a range of chronic diseases.

The different types of GLP-1 drugs

1. Liraglutide (Saxenda)

Good for low levels of weight loss

A daily rather than weekly injection, Saxenda isn’t quite as powerful or as long-lasting as the newer alternatives, which has led to it being eclipsed.

“There’s been two or three generations of these drugs,” says Professor Wilding. “The initial ones have mostly disappeared from the market because they were once or even twice a day injections, and relatively weak compared to what we’ve got now. But Saxenda is still available and has some effect on body weight.”

2. Semaglutide (Ozempic/Wegovy)

Good for building up the dosage more slowly

A weekly GLP-1 injection, semaglutide is marketed as Ozempic for the treatment of Type 2 diabetes and Wegovy for the treatment of obesity.

Wilding explains that the main difference between Ozempic and Wegovy is that the latter offers a slightly higher maximum dose of semaglutide which people can work their way up to using. “For diabetes, the dose usually only goes up to one milligram,” he says. “But with Wegovy, the maximum dose is 2.4mg.”

Compared with Saxenda, it’s not only longer-acting but the reach of the drug also seems to be more potent. “We know from some of the preclinical studies in mice that semaglutide seems to reach parts of the brain that liraglutide doesn’t reach,” says Wilding.

3. Tirzepatide (Mounjaro)

Good for higher amounts of weight loss

The most powerful weight-loss drug on offer, Mounjaro is slightly more complex than either Saxenda or Wegovy. As well as activating the body’s GLP-1 receptors, it also stimulates receptors used by a second gut hormone called GIP.

“What the gut hormone is doing is helping the GLP-1 work better,” says Martin Whyte, an associate professor of metabolic medicine at the University of Surrey. “So it’s almost like a super GLP-1.”

Wilding points out that Mounjaro also comes in six different doses, with the 5mg dose offering similar weight loss benefits to Wegovy, and then the higher 10mg and 15mg doses helping patients to lose even more weight.

4. Retatrutide

The most powerful weight-loss drug

This isn’t yet clinically available, but retatrutide already created something of a stir in the obesity medicine world on the back of clinical trials which suggest that it’s the most powerful weight-loss drug created so far.

Developed by Eli Lilly, makers of Mounjaro, you’ll hear retatrutide described as a “triple agonist”, which means that it mimics the effects of three gut hormones: GLP-1, GIP and another one called glucagon. This means that it doesn’t just blunt appetite, it also speeds up the metabolism of body fat.

Currently being studied in a phase three trial, it could feasibly hit the market in the next couple of years. There’s more expected to come as well, with more than 100 different drugs based on various gut hormones in different stages of clinical development.

“We’re going to see a lot of competition and interest in this area, because now people know it can be done,” says Wilding. “That’s probably a good thing, because it means more choice for patients. Maybe somebody who doesn’t tolerate one will be able to take another one, so having a range of options available is good.”

Which GLP-1 drug is the best for weight loss?

The following table shows the average percentage weight loss for each GLP-1 drug:

Dr Whyte says that when he treats UK patients with obesity, he now almost always prescribes Mounjaro because the weight loss effects are clearly superior to anything else currently available.

“In terms of weight loss, there’s a very good argument to just use tirzepatide and not semaglutide full stop,” he says.

This could change again if retatrutide is approved for mainstream use in the coming years. However, Whyte predicts that the emergence of more options will allow doctors to make more tailored choices.

“In many individuals, you don’t need 20% weight loss,” he says. “The data shows that if your BMI is less than 35, something like 7% weight loss can already achieve a clear metabolic benefit. So something like semaglutide would be perfectly reasonable and would do the job just as well.”

What are the other health benefits?

GLP-1 drugs have also been linked to other health benefits besides weight loss. They have been shown to help reduce the risk of:

• Sleep apnoea
• Addiction
• Cardiovascular disease
• Chronic kidney disease
• Obesity-related cancers
• Non-alcoholic fatty liver disease
• Dementia

From curbing alcohol addictions to reducing the risk of Alzheimer’s disease, GLP-1s have the potential to become game-changing medications for a plethora of other conditions.

Some benefits are, of course, a direct result of weight loss. Tirzepatide is now approved by regulators in the United States as a treatment for obstructive sleep apnoea, a dangerous midlife condition exacerbated by excess weight where breathing stops and starts dozens or hundreds of times during sleep, while research presented at the 2025 European Congress on Obesity showed that GLP-1s can almost halve the risk of obesity-related cancers.

Semaglutide is also officially recognised by regulators as reducing the risk of heart attack and stroke in people at higher risk of these conditions.

“We see improvements in things like cholesterol and other fats in the blood [when people take GLP-1s],” says Wilding. “Bad fats tend to go down and good fats tend to go up. We see lowering of blood pressure and general improvements in quality of life.”

But research is also showing that GLP-1s can have even broader effects, curbing cravings for alcohol and other substances in people with long-term addictions, something which scientists believe relates to the impact of these drugs on reward pathways in the brain.

Most fascinatingly of all, clinical trials are now formally testing whether these drugs can protect against the onset and progression of dementia in people with mild cognitive impairment, with studies in animals suggesting they can reduce inflammation in the brain and stimulate the repair of damaged nerve cells.

What are the side effects?

As with most drugs, there are downsides. These include:

• Nausea
• Constipation or diarrhoea
• Gallstones
• Pancreatitis
• Hypoglycaemia
• Allergic reactions

Wilding says that the majority of people who take GLP-1 drugs can expect some side effects, most commonly nausea and gastrointestinal (GI) problems such as constipation or diarrhoea. In a small number of cases, these issues can prove so intolerable that users end up quitting the medication, but the majority of users are able to cope by slowly tapering up the dose over time.

“If you look at the trials, 80-90% of the people are able to stay on the medicine, but there are a significant minority who can’t, just because of these GI side effects,” says Wilding.

Much rarer problems can also occur, such as the formation of gallstones, which Wilding says can be a side effect of any form of weight loss, as well as pancreatitis or inflammation of the pancreas. “This can be a very serious medical condition which can be related to gallstones,” he adds.

Because of the risk of side effects, researchers are keen to emphasise that these drugs are not benign and should be used with caution. In April 2025, researchers from the US Centres for Disease Control and Prevention assessed data from thousands of emergency department visits which had been directly linked to semaglutide.

While gastrointestinal side effects were the main reason, the data showed that some patients also experienced allergic reactions to the drug and a sudden drop in blood sugar levels, known as hypoglycaemia, which in some cases required hospitalisation.

“It may be that semaglutide interacts with other medications, or that some people who use semaglutide stop eating to the point of severe hypoglycaemia,” says Dr Pieter Cohen, an associate professor at Harvard Medical School and one of the study authors.

According to Whyte, side effects are less common with tirzepatide and there is a hope that this will also be the case with retatrutide.

How available are GLP-1 weight-loss drugs?

Wegovy is available for the first time in New Zealand from tomorrow, July 1.

Medsafe approved Ozempic for diabetes in 2023 and Wegovy for weight loss in March 2025 as a prescription medication in conjunction with a reduced-calorie diet and increased exercise.

Wegovy’s guidelines state that the medication is for those considered obese and with a BMI over 30, or people with a BMI of more than 27 who have at least one weight-related comorbidity, such as Type 2 diabetes. It can also be prescribed to adolescents who are considered obese.

How much does it cost?

While Medsafe approved the drug, Pharmac is currently not funding it, so users will pay out of pocket and the cost is high and ongoing.

Wellington pharmacist Clive Cannons told RNZ he estimated Wegovy will cost users between $450 to $600 a month. It will be determined by the wholesale price and the price structure of individual pharmacies, he says.

Dr Lara Courtenay, a weight loss physician at the MacMurray Centre in Auckland, believes the price will be around $500 a month, similar to the AU$460 Australians typically pay per month.