Over the last decade, several epidemiological studies have been published on the oral contraceptives (OC) and cancer risk issue. These studies were reviewed in June 1998 by an IARC Working Group, and are summarised in the IARC monograph 72 .
A collaborative reanalysis of individual data on 53.297 breast cancer cases and 100.239 controls indicated that there is a moderate excess risk for this disease among current or recent OC users, which tends to level off in the few years after stopping use. OC use has also been found to be positively associated with cervical cancer risk in HPV-positive women. Conversely, OC (with the exception of the currently not used sequential type) reduce the risk of endometrial cancer. Further, data on ovarian cancer indicate a long lasting protection from OC use, which may well be evident up to 20 years after cessation. Several studies have suggested an inverse relation between use of OC and risk of colorectal cancer, but no association with duration of use was observed. An increased risk for OC users of hepatocellular carcinoma is considered as established. The main established evidence on the OC and cancer issue can be summarised as follows:
• There is a small increased risk of breast cancer among current users, but not among former users who have ceased for 10 or more years.
• OC lower the risk of endometrial and ovarian cancer, and the protection seems to persist after cessation of use.
• A reduced risk of colorectal cancer among OC users is possible, but this issue is still open to discussion.
• OC are related to increased risk of cervical cancer and liver cancer, but the public health importance of these associations is small in developed countries.
• OC have been used for 40 years, and the formulations have been modified repeatedly. It is therefore difficult to propose further modifications that may appear favourable on the risk of selected diseases without increasing the risk of other side effects.