A study carried out by the World Health Organization has produced the first compelling evidence of a long suspected link between the contraceptive pill and cervical cancer. The research, carried out by the WHO's International Agency for Research on Cancer, found that prolonged use of the oral contraceptive pill increased the risk of cervical cancer up to fourfold, but only in women who carry the human papillomavirus (HPV).
The agency's Multicentric Cervical Cancer Study Group reviewed eight studies from Thailand, the Philippines, Morocco, Brazil, Peru, Paraguay, Colombia, and Spain. Their results are published in last week's Lancet (2002;359:1085-92). Women who had taken the pill for 5-9 years were nearly three times more likely than non-users to develop cervical cancer. Women who had taken the pill for more than 10 years were four times more likely than non-users to develop the disease. These risks did not vary with time since first or last use, or by age at first use.
The study considered only women who were infected with HPV, because the authors took the generally accepted view that the virus is “probably a prerequisite for the development of the disease.” Over 99% of women in Britain in whom cervical cancer is diagnosed test positive for DNA of HPV, an infection that affects a third of all women in their twenties.
Another study by the same team, also in last week's Lancet (2002;359:1093-101), confirmed the widespread belief that multiple pregnancies are another risk factor for cervical cancer. Women who tested positive for HPV and who had had seven or more full term pregnancies were 3.8 times more likely to develop the disease than infected nulliparous women.
“These factors appear to act as independent multipliers of risk,” said Silvia Franceschi, one of the studies' authors. HPV positive women who had had more than five full term pregnancies and had taken the pill for more than five years ran nearly 12 times the risk of cervical cancer as HPV positive nulliparous women who had never taken the pill.
“We don't have data on the types of pill being used, and it is possible they were at higher doses than would be typical in Europe today,” said Dr Franceschi. “Some of the original studies dated from the late 1980s. But from other studies we would expect a similar effect from progesterone based preparations.”
Cervical cancer is the most common cancer among women in many developing countries. The overall lifetime risk is about 5% in parts of Africa, India, and Latin America, compared with 1% in Europe and North America. Yet HPV infection is extremely common throughout the world. In the United Kingdom the virus is present in perhaps a third of all women in their twenties.
In British women about 3000 cases of cervical cancer are diagnosed each year, and 1300 women die from it. Both the incidence and mortality have fallen considerably since the introduction of the Papanicolaou smear test.
The Family Planning Association sought to allay fears about using the pill. “The overall likelihood of getting cervical cancer in the UK is low, whether you use the pill for a long time or not,” said a spokeswoman. “The benefits of using oral contraception outweigh the risks for the vast majority of women. The pill reduces the risk of cancer of the ovaries and of the womb.”
Professor Gordon McVie, director of Cancer Research UK, also urged women not to worry unduly. “It is important they attend their screening appointments, which will pick up any changes long before they develop into cancer.”