Antibiotic use predicts an increased risk of cancer.

Antibiotic use has been hypothesized to be associated with the risk of cancer but the evidence is sparse and inconsistent. The aim of the present study was to determine whether antibiotic use predicts the development of various cancers. This nationwide cohort study included 3,112,624 individuals, aged 30-79 years, with no history of cancer. Information on their antibiotic use between 1995 and 1997 was obtained from the Drug Prescription Registry. During the period 1998-2004, 134,070 cancer cases were ascertained from the Finnish Cancer Registry. Cox proportional hazards regression was used to estimate the relative risks (RRs) with 95% confidence intervals (95% CIs). Antibiotic use was associated with an increased risk of cancer: for categories of increasing antibiotic use (0-1, 2-5 and >/=6 prescriptions), RRs (95% CIs) for cancer were 1.0 (reference), 1.27 (1.26-1.29) and 1.37 (1.34-1.40). RRs (for comparison of lowest and highest exposure group) for the most common primary sites i.e. prostate, breast, lung and colon were 1.39 (1.31-1.48), 1.14 (1.09-1.20), 1.79 (1.67-1.92), and 1.15 (1.04-1.26), respectively. RRs for other primary sites varied between 0.90 (0.76-1.05) for ovary to 2.60 (1.60-4.20) for endocrine gland (excluding thyroid). In conclusion, antibiotic use predicts an increased risk of cancer. Because of the design of our study the possibility of residual confounding cannot be excluded and further studies are required to confirm the results.