Taking antibiotics in the month before starting immunotherapy dramatically reduces a cancer patient’s chances of survival, The Guardian reports citing a small but groundbreaking study.

Scientists at Imperial College London believe antibiotics strip out helpful bacteria from the gut, which weakens the immune system. This appears to make it less likely that immunotherapy drugs will boost the body’s cancer-fighting capability.

In their study of nearly 200 cancer patients in two NHS hospitals, the researchers found that those who had taken broad-spectrum antibiotics for just a few days for common problems such as chest infections survived for a median of two months after immunotherapy, compared with 26 months for those who had not been on antibiotics.

The substantial difference in survival, and clear evidence from CT scans that tumours grow more rapidly in those who have taken antibiotics, has led the researchers to call for more work to be done urgently to guide doctors.

“Antibiotics clearly wipe out some of the gut microbiota,” said Dr David Pinato, from ICL’s department of surgery and cancer, who was one of the authors of the study published in the journal Jama Oncology. “If you have got a good microbiome, you are more likely to have educated your immune system to fight cancer better.”