The WHO Collaborative Study of Neoplasia and Steroid Contraceptives: the influence of combined oral contraceptives on risk of neoplasms in developing and developed countries.

A hospital-based case-control study was conducted in eight developing and three developed countries to determine whether use of combined oral contraceptives alters risks of various cancers. An observed trend of increasing risk of invasive cervical cancer with duration of use may not represent a causal relationship and is the subject of further study. Decreased risks of ovarian and endometrial carcinomas in users likely indicate a protective effect of oral contraceptives, the degree of which was similar in developing and developed countries. A small increase in risk of breast cancer in recent and current users was found to be somewhat greater in developing than developed countries. Both causal and non-causal interpretations of this finding have been offered. No associations were found between oral contraceptives and in situ cervical, hepatocellular, cholangio, or gallbladder carcinomas, or uterine sarcomas; but the power of this study to detect alterations in risks of these neoplasms in long-term users was low.