Women who use birth control pills for a long time are more likely to develop cervical cancer, according to a new study.

Researchers from the International Agency on Cancer Research in Lyon, France, found that the risk increased by more than 50% after five years of oral contraceptive use, and more than doubled after 10 years. The study was published in The Lancet (Vol. 361, No. 9364: 1159-1167).

Cervical cancer is a relatively uncommon cancer in the United States. It will be diagnosed in about 12,000 American women in 2003, according to American Cancer Society estimates, and it will kill about 4,000 women. Cervical cancer is not among the top 10 cancers in American women, mainly because of early detection through Pap tests.

However, it is the second most common cancer in women in less-developed countries and a major cause of death worldwide. A major risk factor is infection with the human papilloma virus (HPV). The HPV strains that cause cervical cancer are sexually transmitted and very common.

Another risk factor that has been described by many researchers is the use of birth control pills. The World Health Organization commissioned scientists from the International Agency for Cancer Research and the United Kingdom to review the evidence that birth control pills increase the risk of cervical cancer.

The researchers, led by Jennifer Smith, PhD, re-analyzed 28 published studies of more than 12,000 women with cervical cancer. All these studies looked at the birth control pill use of the women with cervical cancer compared with that of women who didn’t have cervical cancer.

Women who used the pill for less than five years had no increase in their risk of cervical cancer. But women who reported using birth control pills from five to 10 years had a 60% increase in their rate of cancer; women using the pill more than 10 years had a rate over twice that of women who didn’t use the pill.

One key question that remained unanswered was how long a woman had to be off the pill before her risk of cervical cancer dropped. The researchers estimated that the risk fell after a woman was off birth control pills for more than eight years. But they confessed to being uncertain because there haven’t been enough studies.

The study did not examine why oral contraceptives might be linked to an increased risk of cervical cancer. Some have suggested that women on birth control pills have a higher risk because they are more likely to be sexually active and more likely to be infected with HPV. But, when the researchers focused only on women who had HPV infection, the risk from birth control pills was still as high.

When they examined women who had other risk factors for cervical cancer, such as smoking, having a high number of sexual partners, or living in less-developed countries, the risk was always higher in women taking birth control pills for a long time.

But women should not be unduly alarmed by the findings, said Debbie Saslow, PhD, director of breast and gynecologic cancer for the American Cancer Society.

Oral contraceptives can also decrease the risk of some other cancers, such as ovarian cancer, she noted, and are considered to be very safe. “Women should consider that there are lots of pros and cons to every birth control method and choose what they are most comfortable with,” she said.

And all women, Saslow emphasized, should follow ACS screening recommendations for cervical cancer, regardless of whether they use oral contraceptives. The Society recommends women get a traditional Pap test every year (or a liquid-based Pap test every two years) until age 30. After that, women with three normal Pap tests in a row should get some type of Pap test every two to three years. Or, women over 30 can opt for a Pap test plus a test for HPV every three years.

“Fortunately, we have excellent screening and can detect and remove or treat the vast majority of changes in the cervix before they become cancerous,” Saslow said.