How do weight loss jabs work? Dr Amir Khan reveals all you need to know

In recent years, little else has sparked such big opinions in the world of health as weight loss jabs - and the chatter isn't going anywhere as Serena Williams has become the latest celebrity to reveal she's taking the medication.

The 23-time Grand Slam champion and four-time Olympic gold medallist appeared in an advertisement for Ro, a healthcare company her husband invests in, to reveal that weight loss jabs helped her lose 31 pounds after the birth of her second child. She didn't confirm whether she was taking Mounjaro, Zepbound, or Ozempic for weight loss - three drugs available in the US - but said: "They say GLP-1s for weight loss are a shortcut. It's not, it's science. After kids, it was the medicine my body needed."

It was a statement that had millions of women teetering between curious and confused. A top-level professional tennis player on weight loss medication? If someone of her fitness background and access thinks that's the only way forward, should it be a green light for the rest of us?

No, says Dr Amir Khan, a GP with a special interest in diabetes management, who is a frequent medical expert on ITV, and woman&home's own resident doctor. Speaking on Lorraine, he said: "People are looking at these drugs almost like Smarties. They go on it for a short period of time, for a holiday, a wedding, or something like that. These are really serious medications."

We caught up with Dr Amir Khan to get the latest details on weight loss jabs in 2025 and what you need to know if you were considering them for yourself.

What are weight loss jabs?

In the UK, Mounjaro, Wegovy, and Saxendar are three medications licensed for weight loss. They are the brand names for tirzepatide, semaglutide, and liraglutide. "Though they all have different names, they all contain variations of the same hormone, GLP-1, which stands for glucagon-like peptide-1," says Dr Khan. "It's a hormone your gut naturally produces after eating."

Out of the predicted 1.5 million people in the UK who take weight loss jabs, an estimated half take Mounjaro, and nine out of 10 of these prescriptions are private. "This [medication] is slightly different as, alongside GLP-1," the doctor explains. "It also contains another hormone called glucose-dependent insulinotropic (GIP), which accounts for its superior effects over the other two brands. It's often referred to as the 'King Kong' of weight loss drugs."

All three drugs (and orlistat, a weight loss drug that works slightly differently from GLP-1s) are available on the NHS for those who meet tight eligibility requirements. Those who aren't eligible for NHS support but still have a body mass index (BMI) of 30 or higher, or 27 or higher with at least one weight-related health condition, can buy the drug privately.

How do weight loss jabs work?

Mounjaro, Wegovy, and Saxendar work in the same way as they all contain an artificial form of GLP-1. Firstly, "they slow down the digestive process by keeping food in your stomach for longer," explains Dr Khan. "This keeps you full and helps control portion size."

Secondly, "they act on the brain's appetite centre in the hypothalamus and switch off your hunger signals and switch on feelings of satiety," he says. "Not only are you not hungry, but you're not constantly thinking of food, something that many of us will know as 'food noise'."

Weight loss jabs were initially created to treat type 2 diabetes as they increase the pancreas's insulin production, which helps to bring down blood sugars.

The expert view

"I am a GP with a special interest in diabetes. I see patients all the time who are struggling to get to a healthy weight," says Dr Khan. "People who do not struggle with their weight often think of those who do as lacking willpower. But living in a larger body is complex.

"Several factors govern weight. Yes, what you eat and certain types of exercise can help you manage your weight, particularly resistance training. However, genetics, your relationship with food as a child, medical conditions, menopause, medication and affordability, and access to good, nutritious food and spaces to exercise in also influence weight.

"Sadly, I do feel those who live in larger bodies are judged for their weight. Then, if they do choose to access these drugs for weight loss, they are judged a second time for using a 'quick fix', and not 'earning' their weight loss.

"I think it's important nobody is judged for taking these drugs, and women in particular. Women have had to live their entire lives alongside social scrutiny of their bodies. Through insidious messaging online, on television, and in the press, many believe that their self-worth is tied up in how they look, or more accurately, how they believe they should look.

"Adding to this societal pressure is the fact that many celebrities, including Whoopi Goldberg, Stephen Fry, and Serena Williams, have admitted to taking these drugs to help them lose weight. Now, I don’t have an issue with anyone taking these drugs for their own personal reasons, and it is good that they admit to taking them, so people know how their dramatic weight loss has happened. What I do take issue with is celebrities who are promoting the drugs to their fans.

"Serena Williams, one of the best athletes the world has ever known, and someone who I am a big fan of, said she took Mounjaro to help her shed 31lbs after she had her babies. Perfect, good for her. No judgment from me. What makes me uneasy is the fact that she advertised the drug on her social media accounts, and that her husband is involved in the company that sells the drugs. There is a real conflict of interest there.

"To be clear, this is not a man judging a woman for taking control of her body. Like I say, I am happy for Serena to do what she wants with her own body. But, if she is doing an advert for the drug, she should make it clear that both she and her husband stand to gain financially from the sale of the drugs from that particular website.

"My feeling is the focus should always be on health and not weight. You can be cardiovascularly fit and have a high BMI, and conversely, I have seen many people with lower BMIs with multiple metabolic-related health issues.

"What is vital is that when people choose to take these drugs, they are making an informed choice. They are, after all, medications, and all medications come with risk, and you should only ever take them if the benefits outweigh those risks," he says.

Side effects of weight loss jabs

It's important that "when people choose to take these drugs, they are making an informed choice," says Dr Khan. "All medication comes with risk, and you should only ever take it if the benefits outweigh those risks."

The most common side effects of weight loss jabs include:

• Gastrointestinal changes: "Common side effects are bloating, nausea, change in bowel habits and acid reflux," says Dr Khan, as these weight loss jabs slow digestion right down.
• Muscle loss: Rapid weight loss often comes with muscle loss. "For people over the age of 40, this can increase the risk of osteoporosis and, over time, lower muscle mass can slow down your metabolism and affect fitness levels," says Dr Khan. "If you do take these drugs, make sure you do regular resistance training three to four times per week and up your protein intake."
• Serious health problems: Though less frequent than above, the side effects of weight loss jabs like Mounjaro include gallstones, kidney issues, eye problems, an inflamed pancreas, and (in animal studies only) thyroid cancer.

Who should avoid weight loss jabs?

Anyone with a history of serious health problems, including kidney issues, gallstones, and pancreatitis, should stay "well clear" of these drugs, adds the doctor. "For you, the risks outweigh the benefits."

It's also important to think about what happens when you stop taking Mounjaro and other weight loss drugs with side effects to this as well.

Per NHS advice, those with a BMI under 30 (or 27 with a weight-related health issue) should not be prescribed weight loss medication.