From ‘Ozempic breath’ to hair loss: sorting the science from the hype about weight loss jabs

Do you gain weight once you stop using them? Do they make your face saggy? Can they improve your mental health? Experts explain whether 13 common beliefs about GLP-1s are true or false

FALSE “GLP-1 medications mimic Glucagon-like peptide-1 – one of the hormones involved in appetite regulation. It doesn’t completely suppress hunger, but it is powerful,” says Dr Federica Amati, nutrition lead at Imperial College School of Medicine and author of The Appetite Reset. “The medicine is about 1,000 times stronger than what we naturally produce from our gut,” she explains. As food passes through the gut, cells called L-cells sense the nutrients, which sparks the production of the GLP-1 hormone. Without the drugs, these bursts of GLP-1 production last a few minutes. “It’s a bit like a sparkler. In comparison, the drugs are like a bonfire that roars for a week,” says Amati. Because the GLP-1 hormone stays in your body for longer if you are on the drugs, you feel fuller faster and your appetite gets suppressed, but you will eventually get hungry.

TRUE “Nausea, constipation, diarrhoea, bloating and vomiting are well-recognised side-effects,” says Dr Adrian Brown, associate professor and specialist dietitian at University College London. “They tend to occur during dose escalation, and most symptoms improve over time.” Eating smaller meals, staying hydrated and limiting fatty foods can help.

“One of the best ways to prevent these side-effects is to prepare your gut, liver and pancreas before you start taking the medications,” says Amati. “If you go into treatment with existing reflux or constipation, it’s only going to get worse.” She advises eating more plants and gradually upping your fibre intake. Once taking the drugs, never push past the point of fullness. “The gastric emptying is so slow that you need to stop straight away.”

PARTLY TRUE The drugs themselves don’t make you want to drink less, but people can experience this as a side-effect. This is partly because we get a lot of water from food, especially fruits and vegetables. “These drugs massively reduce the amount that you eat – up to 40% reduction in calories,” says Amati. You may also drink less if you’re experiencing nausea, constipation or reflux. “It’s important that you do drink, because dehydration is one of the key reasons why reflux and constipation may be worse,” she says. As well as drinking water, she recommends having hydrating foods and smoothies, especially on the day you take the drugs (typically, the injections are used once a week, while the pills are taken daily), when you can have the most appetite suppression.

TRUE Taking GLP-1 medication can “substantially reduce food intake, and that creates the potential for nutrient deficiencies if the quality of a person’s diet is poor,” says Brown, who has researched this area. He found that some people on these drugs may not get enough vitamin D, vitamin B12, iron, calcium and protein.

“If your diet was relatively healthy to begin with, that’s fine because you’re just eating less. But this is not true for many people,” says Giles Yeo, professor of molecular neuroendocrinology at the University of Cambridge and an obesity expert.