Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Antibiotics are commonly prescribed to patients undergoing traditional chemotherapy, as the treatment weakens the body, making it more vulnerable to bacterial infection.However, immunotherapy, hailed as the future of cancer drugs, relies on a lively pool of bacteria in the gut to produce friendly T Cells to take on the cancer.Dr Matthew Krebs, a consultant in experimental cancer medicine at The Christie, who co-authored the study, said: “People see their GP and the GP thinks, ‘Oh my goodness it’s a cancer patient, they need antibiotics’.“If someone genuinely has a need, then of course we should prescribe them antibiotics.“What we’re saying is think really carefully about it.”Immunotherapy drugs work by prompting the body’s immune system into recognising and fighting cancer cells.The family of treatments is currently routinely available for NHS patients suffering from forms of lung and bladder cancers and melanoma, although it is expected to become the standard treatment for many more categories of patient in the coming years.A study published in the New England Journal of Medicine in April even suggested that using immunotherapy before undergoing surgery to remove tumours significantly reduced the chances of the disease returning.However, currently only around 20 per cent of patients respond to immunotherapy and The Christie researchers, funded by Cancer Research UK, began their research because they believed antibiotics may be partly responsible for the failure.The new study, presented at the American Society of Clinical Oncology (Asco) meeting in Chicago, is the first large clinical analysis to show a definitive link. Nadina Tinsley, clinical fellow and lead author, said the greater the number and diversity of bacteria in the gut, the more T cells there are available to take on cancers, but a course of antibiotics can suppress bacteria levels for weeks.“I think potentially it’s quite a big problem,” she said. “Clearly antibiotics are a really important part of patient management and we need to treats serious infections and prevent life-threatening infection, even death.“But the challenge is striking the right balance and making sure that we identify those patients that are at risk of having a serious infection, without giving antibiotics for less justified indications and maybe overusing antibiotics.”Dr Sumanta Pal, an Asco expert and authority on immunotherapy, described the new NHS study as the “most robust” to date, adding: “It ties into a theme of really not using antibiotics for frivolous or non-indicated uses,” he said. GPs overprescribing antibiotics is significantly damaging the survival chances of cancer patients, leading oncologists have warned.A major new NHS study has found that sufferers undergoing the latest cancer treatments survived for only half as long if they were also taking the common infection-fighting drugs.Family doctors have been warned to “think really carefully”, before prescribing antibiotics after the analysis of more than 300 patients at the Christie Hospital in Manchester concluded the drugs were wiping out gut bacteria crucial for fighting cancer.The warning comes amid escalating concern that the profligate use of antibiotics by doctors has fuelled the rise of lethal drug-resistant superbugs.Researchers analysed data from 303 patients with melanoma, renal and non-small cell lung cancer being treated with immunotherapy drugs, known as checkpoint inhibitors, between 2015 and 2017.Survival rates among patients who took antibiotics – at any stage from two weeks before their cancer treatment started, to six weeks after treatment – were compared with those of patients who took none.The antibiotic group survived for an average of 317 days, while those who had not taken antibiotics survived for 651. Meanwhile those who had used antibiotics over a longer period or been given multiple types of the drug had an even lower survival span of just 193 days.