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The longer a woman takes birth-control pills, the lower her risk of developing ovarian cancer, according to a new study.
The research, published in today's edition of the medical journal The Lancet, shows that women who take oral contraceptives for a decade can slash their cancer risk by more than one-third.
By comparison, taking the Pill for up to four years reduces the risk of ovarian cancer by about 22 per cent.
Women who took birth control pills for more than 15 years saw their risk fall by 58 per cent.
On average, women used the Pill for about five years.
Wylam Faught, medical adviser for Ovarian Cancer Canada, said the protective effect of birth-control pills was known among specialists in the field, but the new research should help get the message out to the public.
"The Pill is the most effective tool we have," he said in an interview.
"This study confirms that there is something, something pretty simple, that can protect against ovarian cancer. And that's good news."
The new research does not specifically examine the mechanics of how birth-control pills reduce the risk of ovarian cancer.
But Dr. Faught, who is also the chairman of the department of obstetrics and gynecology at the University of Ottawa, said that it is almost certainly through the suppression of ovulation. Pregnancy - and hence fewer menstrual cycles - also lowers the risk of ovarian cancer.
Both oral contraceptives and pregnancy alter the body's ratio of the female sex hormones estrogen and progestin; and, in laboratory tests, higher progestin levels have been shown to reduce the risk of ovarian cancer.
The new study is a meta-analysis: a compilation and analysis of previously published research.
Valerie Beral and colleagues at the Cancer Research UK epidemiology unit at Oxford University in England brought together 45 studies conducted in 21 different countries including Canada, the United States, Sweden and China.
In total, the data included 23,257 women with ovarian cancer (31 per cent of whom had used birth-control pills). They were compared to a control group of 87,303 women without ovarian cancer (37 per cent of whom had used birth-control pills). Dr. Beral and her team found that use of oral contraceptives over a period of a decade or more markedly reduced the incidence of ovarian cancer before age 75 - to a rate of 8 per 1,000, compared to 12 per 1,000 among women who did not use the Pill.
Similarly, the death rate from ovarian cancer fell notably, to 5 per 1,000 among users of birth-control pills from 7 per 1,000 among those who did not take the Pill.
"Use of oral contraceptives confers long-term protection against ovarian cancer," Dr. Beral said.
Eduardo Franco, director of the division of cancer epidemiology at McGill University, called the findings "unequivocal good news."
In a commentary also published in The Lancet, he said the study - in which he was not involved - "makes a major contribution to our understanding of the role of oral contraceptives in the causation or prevention of ovarian cancer."
What remains unclear, however, is whether women should take oral contraceptives specifically to reduce their risk of ovarian cancer.
Taking birth-control pills slightly increases the risk of breast cancer, but only while a woman is taking the Pill.
The benefits of taking oral contraceptives to reduce ovarian cancer, however, seem to be more lasting, though they drop off after age 75, the study noted.
According to the paper, more than 100 million women worldwide routinely use oral contraceptives.
The study estimates that, as a result, about 30,000 cases a year of ovarian cancer will be averted, and that number could rise substantially as use of oral contraceptives increases in the developing world.
In Canada, it is estimated that 2,400 women were diagnosed with ovarian cancer and 1,700 died from the disease in 2007. It is one of most lethal cancers, in large part because it is difficult to diagnose.
There is no reliable screening test or vaccine for ovarian cancer, and little is known about how to prevent the disease. Symptoms are also notoriously vague, earning it the moniker "the cancer that whispers."
