A Working Group comprising 21 scientists from 8 countries was convened by the IARC Monographs Program of the International Agency for Research on Cancer (IARC), part of the World Health Organization.

The Working Group stresses that there also is convincing evidence that oral contraceptives have a protective effect against some types of cancer, and advises each woman who uses these products to discuss the overall risks and benefits with her doctor.

Contraceptive Risks: Up ...

"These new IARC Monographs address exposures that are experienced daily by many millions of women worldwide," says Dr. Peter Boyle, Director of IARC. "It is of enormous public health importance that we identify and understand the full range of effects of these products."

More than 100 million women -- about 10% of all women of reproductive age -- currently use combined hormonal contraceptives. Approximately 20 million women in developed countries were using hormonal menopausal therapy at its peak around the year 2000.

There is a small increase in the risk of breast cancer in current and recent users of oral contraceptives, notes the Working Group. However, ten years after cessation of use, the risk appears to be similar to that in never-users.

The risk of cervical cancer increases with duration of use of combined oral contraceptives. The risk of hepatocellular carcinoma is greater in long-term users of combined oral contraceptives in populations with low prevalences of hepatitis B infection and chronic liver disease -- two major causes of human liver cancer.

... And Down

In contrast, the risks of endometrial and ovarian cancer are less in women who use combined oral contraceptives. The reduction appears to be greater with longer duration of use, and some reduction persists at least 15 years after cessation of use, according to the Working Group.

Because use of combined estrogen-progestogen contraceptives increases some cancer risks but decreases others, it is possible that the overall net public health outcome may be beneficial.

Still, a rigorous analysis is required to demonstrate this, the Working Group recommends. This should be done on a country-by-country basis, and it also should consider the effects on non-malignant diseases.

Assessing Menopausal-Therapy Risks

Epidemiological studies consistently demonstrate an increased risk of breast cancer in combined menopausal-therapy users.

Largely confined to current or recent users, the risk increases with duration of use and exceeds that in women taking estrogen-only therapy.

Endometrial cancer risk depends on the number of days that progestogens are included in the combined therapy, explains the Working Group. When progestogens are taken fewer than 10 days per month, the risk of endometrial cancer is increased, but when progestogens are taken daily, the risk is similar to that in women who never used hormonal therapy.

There was not sufficient evidence to conclude that hormonal therapy has a protective effect at any cancer site, observes the Working Group..

Implications for Patients

Previously, combined oral contraceptives had been determined to be carcinogenic to humans -- but primary liver cancer was the only form specifically implicated. However, combined oral contraceptives alter the risk of several common cancers in women, the Working Group points out. They increase a woman's risk of cervical cancer, breast cancer and liver cancer. At the same time, they have a protective effect against endometrial cancer and ovarian cancer.

Combined menopausal therapy previously was regarded as "possibly carcinogenic to humans." Based on an expanded study base, the new evaluation concluded that it is carcinogenic to humans, increasing a woman's risk of breast cancer and -- when progestogens are taken fewer than 10 days per month -- endometrial cancer.

This new information about cancer risks (and also protection against cancer, in the case of oral contraceptives) makes it important that each woman who uses these hormonal products discuss the risks and benefits with her doctor, the Working Group urges, taking into consideration her personal circumstances and family history of cancer, as well as other diseases.