Antibiotics may significantly shorten the lives of cancer patients receiving immunotherapy treatment, a UK study has shown.
Taking the pills to treat minor infections could affect the length of survival, it was warned, as GPs and oncologists were urged to prescribe with caution.
The researchers, from the NHS Christie Hospital in Manchester, said a balance must be struck between preventing serious infection in cancer patients and avoiding overuse of antibiotics.
The study, presented at the American Society for Clinical Oncology (ASCO) annual meeting in Chicago, is thought to be the largest and "most robust" of its kind.
Researchers analysed data from 303 patients with melanoma, renal and non-small cell lung cancer, who were treated with immunotherapy drugs "checkpoint inhibitors" at Christie NHS Foundation Trust between 2015 and 2017.
Survival rates among patients who took antibiotics - at any point from two weeks before their immunotherapy started to six weeks after the treatment finished - were compared with patients who did not take any.
The antibiotic group lived for around 317 days while those who had not taken them survived for 651 days, the study found.
Patients who had used antibiotics over a longer period or been prescribed multiple courses lived for just 193 days.
Lead author Nadina Tinsley, clinical research fellow, said: "Clearly antibiotics are a really important part of patient management and we need to treat serious infections and prevent life threatening infection, even death.
"But the challenge is striking the right balance between 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."
Co-author Matthew Krebs, consultant in experimental cancer medicine at the Christie Hospital, added: "If someone is genuinely got a need, then of course they should receive antibiotics.
"What we are saying is, think really carefully about it before you prescribe."
Immunotherapy drugs work by stimulating the immune system to recognise and fight the cancer.
The treatment is only effective for an estimated 20 per cent of patients, the researchers said, but scientists do not fully understand why.
Previous studies have suggested having more bacteria in the gut can improve how a patient responds to treatment.
