Abstract
An analysis of the occurrence of breast cancer in this long-term prospective cohort study shows a significant relative risk (RR) in women who have ever used oral contraceptives (OC) of 3.33 in women age 30 to 34 years at diagnosis and an RR of 5.88 (P = 0.0011) in women who were parity 1 at the time of diagnosis. In women below the age of 35 years the RR of 2.38 was not significant. There was no increased risk in women over the age of 35 years. A significant trend relating to duration of use was demonstrable in women who were parity 1 in the analysis of both current and ever-users. An analysis by time since stopping OC use revealed a significant trend in all ever-users, but the trends were much steeper in women of parity 1 or aged 30 to 34 years at diagnosis. There was no evidence that the increased rates in OC users were related to the oestrogen or progestogen dose. The 5 year survival rate in users diagnosed under the age of 35 years was significantly poorer than in comparable non-users. It is possible that the increased rates in younger OC users might be due to an accelerated presentation of breast cancer in those women who would otherwise have been diagnosed at a later time. The non-significant excess risk in users under 35 years of age was approximately 1 in 7,000 users per year. The unresolved discrepancies between the results of the published studies make it impossible at the present time to decide whether or not OC use is associated with an increased risk of breast cancer.
Highlights
The Study organisation, the potential biases, and the principles underlying the interpretation of the data have been detailed elsewhere (Royal College of General Practitioners, 1974)
During a 14 month period which started in May 1968, 23,000 women using oral contraceptives and a similar number of controls who had never used oral contraceptives were recruited by 1,400 general practitioners throughout the United Kingdom
The standardised breast cancer rates for current users of oral contraceptives, former users and controls are given in Table I; neither -of the risk ratios (RR) between current users and controls (RR 1.25) or former users and controls (RR 1.21) were significant
Summary
The Study organisation, the potential biases, and the principles underlying the interpretation of the data have been detailed elsewhere (Royal College of General Practitioners, 1974). During a 14 month period which started in May 1968, 23,000 women using oral contraceptives and a similar number of controls who had never used oral contraceptives were recruited by 1,400 general practitioners throughout the United Kingdom. Living as married, and users and their controls were matched for age. At six-monthly intervals from her respective date of recruitment, the general practitioner supplies details about the subject's oral contraceptive use, all newly presenting episodes of illness and other relevant data. For each calendar month in which a subject uses an oral contraceptive, one month is added to the period of exposure of current users. If the woman stops using the pill her subsequent periods of observation are included in the former user group, unless she restarts use, in which case she again contributes to the current users' periods of observation
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