Controversy over the contraceptive pill and its link with breast cancer has been rekindled.

Klim McPherson, professor of public health epidemiology at the Cancer and Public Health Unit, London School of Hygiene and Tropical Medicine, says that recent reassurances may be "seriously misplaced".

He bases his statement on the fact that much of what we know about breast cancer and the Pill is based on studies done when patterns of contraceptive pill use were quite different.

Most of what we know is based on studies of married women who used the Pill to help space the birth of their children. Today, long term use by younger, unmarried women is the norm.

This rather obvious observation could be crucial in the interpretation of the epidemiology, because we know that breast cancer risk is higher for women with the longest time between the onset of menstruation and first full term pregnancy, says McPherson.

Professor McPherson believes that synthetic hormones may not be carcinogenic themselves, but may induce cell changes which increase the risk of cancer.

Although the risk of developing breast cancer before the age of 50 is usually quoted at around 1 in 50, Professor McPherson says that it's impossible to rule out the possibility of a risk as high as 1 in 18 for those who have been on the Pill for 15 years before their first full term pregnancy (J Epidemiol Community Health, 1999; 53: 258-60).