Antibiotic use and the risk of lung cancer.

Antibiotic use has been associated with an increased risk of cancer in epidemiologic studies. We evaluated the association between antibiotic use and the risk of primary lung cancer by conducting a prospective case-control study nested in a cohort of subjects who were 40 to 84 years old in 1995 to 2004, with at least 2 years of enrollment in The Health Improvement Network. There were 4,336 cases of primary lung cancer. A random sample of 10,000 controls was frequency matched to the cases for age, sex, and calendar year of diagnosis. Antibiotic exposure was measured by the total number of antibiotic prescriptions and the cumulative number of days on antibiotics since enrollment. We discounted exposure 1 year before the date of cancer diagnosis. Relative risks (RR) and 95% confidence intervals (95% CI) were estimated using conditional logistic regression. Smoking, chronic obstructive pulmonary disease, respiratory infections, body mass index, and health care utilization were introduced in the model. Compared with subjects with no prescription of antibiotics before the index date, the crude RR of lung cancer was 2.52 (95% CI, 2.25-2.83) among those who received 10 or more prescriptions. The corresponding RR was 1.31 (95% CI, 1.10-1.57) upon adjustment for confounders. We used directed acyclic graphs to illustrate that the observed higher risk of lung cancer among antibiotic users may be due to the increased frequency of infections in patients with subclinical cancer and to shared causes between cancer and infections. Current evidence is insufficient to support or refute a carcinogenic effect of antibiotics.