They are being hailed as a game changer, but what are weight loss injections, how do they are work and are they safe? We've spoken to the experts.
Weight loss drugs haven't been out of the headlines over the past few months and have been hailed as a breakthrough in the fight against obesity.
The medicines watchdog has agreed that some of these drugs are safe and cost-effective for NHS use and a phased roll-out has begun under prescription.
With millions of people potentially becoming eligible to have these jabs, we spoke to the experts to separate the facts from the fiction.
Naveed Sattar: There are three licensed in the UK: liraglutide (Saxenda), a daily injection; semaglutide (Wegovy); and the newest, tirzepatide (Mounjaro), both weekly. They suppress appetite by imitating the effects of naturally occurring chemicals – chiefly GLP-1 (glucagon-like peptide-1) – that tell us when we’re full.
Tirzepatide also mimics the action of GIP (glucose-dependent insulinotropic polypeptide) that enhances blood glucose control. They turn down ‘food noise’ too – the voice in your head urging you to eat that slice of cake.
Natascha Van Zyl: Losing weight isn’t a matter of willpower; powerful hormones that drive hunger and cravings are involved. I have diabetes and went on semaglutide in March 2024. My weight loss has been slow but stable – around 5kg so far. The constant hunger associated with diabetes has gone. You naturally eat less. The jabs provide a bridge, allowing you to focus on making lasting behavioural change.
NS: Tirzepatide seems most effective for weight loss alone. But tirzepatide and semaglutide have added benefits – semaglutide for heart attack and stroke prevention, arthritis, and kidney disease; tirzepatide for sleep apnoea. There’s also evidence they both improve heart failure and liver disease. More trials are ongoing.
NS: On average, tirzepatide has up to 20-25% weight loss, semaglutide around 15%, and liraglutide around 7%. Everyone plateaus – usually at around one to two years. That’s because as you become lighter, you burn fewer calories so eventually calories in match the calories out. Women tend to lose more than men.
Jana Abelovska: It varies from person to person, but usually we would want to see weight loss of at least 5% of body weight after three months.
NS: The idea is to gradually increase the dose over several months. Some people experience mild to moderate side effects – such as nausea, vomiting, diarrhoea and constipation – that decline as the body adjusts (usually one to three months after starting or increasing the dose).
Eating little and often, stopping before you feel full, eating more slowly, chewing thoroughly, avoiding rich, creamy, fatty or spicy foods and not lying down straight after eating can help minimise these. Some people are more sensitive to the ingredients. Around 1 in 8 to 1 in 12 can’t tolerate them, but with slower dose increases, more are able to take them.
NS: They’ve been tested in people in their 70s and 80s, but there are no official guidelines yet. Older people tend to lose slightly less weight. There’s no limit, but I can’t imagine many people in their late-80s or 90s being prescribed them.
The average age of people in the STEP-HFpEF trial, which looked at the effects of semaglutide in people with heart failure, was 70, with a quarter aged over 75. There were benefits in terms of a reduction in worsening symptoms, such as fatigue and breathlessness.
JA: Weight loss medications can be riskier for older people because of the risk of loss of muscle and bone mass as people lose weight, and/or nutritional deficiencies due to poor appetite.
NS: Most people regain weight once they stop but it is usually slow. In one trial, people who took Mounjaro lost 20% of their body weight over 36 weeks, but regained half over the following year.
However, they still weighed 10% less than they would have otherwise. Some people may only need them from time to time. A few who manage to make sizeable lifestyle changes may be able to stop for many years, while others may use them occasionally or at lower doses. For heart disease we’d usually recommend staying on them.
NVZ: Many fear they can’t maintain results alone. Some people may need ongoing treatment and if psychological or lifestyle adjustments can help, we explore those, combining diet, lifestyle, behaviour change, even gut health to achieve lasting success.
NS: We don’t have much data yet as the longest usage we have had in the trials is between three and four years. That said, GLP-1s have been prescribed in lower doses for people with diabetes for around 20 years and generally appear effective and safe.
JA: There is still more research to be done. People who take semaglutide are slightly more likely to develop an eye condition called non-arteritic anterior ischemic optic neuropathy (NAION), which can lead to vision loss.
JA: Semaglutide is prescribed for a maximum of two years. After six months, you’ll be reviewed. If you’ve lost at least 5% of your body weight you may be prescribed it for longer. With liraglutide, we review after three months.
NS: The latest thinking is that there’s no need to come off them, just as we don’t stop taking blood pressure or cholesterol-lowering medication.
NS: Access is limited due to cost and varies by country/region. Semaglutide and liraglutide may be offered if you have a BMI of over 35 and a weight-linked health condition if you are in a specialist weight management programme.
NICE guidance [for England only] on tirzepatide issued in December 2024 said NHS patients could start receiving it within 90 days if they are in specialist weight management services, or 180 days for those at highest risk of weight-linked problems, such as a BMI over 40 or needing weight loss for life-saving surgery. Nine out of ten people currently get them privately.
JA: It depends on the dose and the drug, but £200-300 for a self-injection pen, which provides four weekly doses. You can get your prescription via a GP, or a high street or online pharmacist. You’ll have a consultation to check the medication is suitable for you.
NS: Following stricter rules from the General Pharmaceutical Council in February, you’ll need an in-person or video consultation to verify your BMI, and confirmation from your GP or medical records when buying from an online pharmacy.
NS: The evidence is growing for other benefits partly because we in the medical profession underestimated how much excess weight causes or exacerbates so many health conditions.
While some of the effects are directly related to the molecules and don’t require weight loss, most benefits are linked to the weight loss and eating fewer calories.
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