New research suggests that people who carry variations in genes linked to appetite and digestion can lose more weight when taking drugs to treat obesity such as Wegovy and Mounjaro.
The findings, which were outlined in the Nature journal, could explain why some experience bad side-effects when taking the drugs, and why some lose far more weight than others.
Weight-loss drugs, which remove feelings of hunger by acting like a hormone that makes users feel full, are growing in popularity in the UK.
More than 1.6 million tried them in the past year, and that number is expected to rise, with most being bought privately through online pharmacies as the NHS only offers Wegovy and Mounjaro to a small number of people with obesity and other related health issues.
The amount of weight lost when taking these drugs can vary widely, with drug trials suggesting 14 per cent weight loss on Ozempic and Wegovy, and 20 per cent on Mounjaro.
Experts have said that, as well as genes, many other factors such as your sex and age can also have an impact in influencing how well the drugs work.
The new study, which saw 15,000 people take weight-loss medications, saw them lose an average of 11.7 per cent of their body weight during around eight months of treatment - but some lost 30 per cent while others lost little or nothing.
Everyone involved in the study had also signed up for gene-testing by the company 23andMe, and that data was used to chart the experiences of people taking the drugs, with researchers finding a pattern which suggested a link between some variants and the drugs effectiveness.
Professor Ruth Loos, from the University of Copenhagen, wrote about the research, saying that the study found a genetic variant associated with weight loss, which was also associated with nausea.
"People lose more weight if they have this variant," she said, with the extra weight lost coming to an average of around 1.6 lbs.
The variant is high in people with European ancestry, as 64 per cent carry one copy, compared to seven per cent of African Americans - but 16 per cent carry two copies, and those people can double the amount they lose.
Another variant was identified in the study which could be responsible for one per cent of people taking Mounjaro experiencing particularly bad experiences of vomiting - almost 15 times worse than normal.
Dr Marie Spreckley, from the University of Cambridge, said that genetics was only one part of a much more complex picture, and that the main drivers of outcomes were "behavioural, clinical, and treatment-related factors".
Previous research suggests that women are more than twice as likely to lose 15 per cent of their body weight on Mounjaro than men, while being younger, white or Asian have also been linked to more weight loss.
Professor Naveed Sattar, metabolic health expert from the University of Glasgow, said: "Overall, these findings are scientifically interesting, but they are a long way from changing clinical practice.
"What we really need now is more robust trial data to better define the balance of benefits and harms with these and many other emerging newer therapies."
