THE Catholic church is facing controversial calls to allow nuns to take the contraceptive pill to protect them from cancer.

Nuns have a much higher chance of developing breast, ovarian and uterine cancers because their life of celibacy means they don't have children.

Australian cancer experts Dr Kara Britt and Professor Roger Short believe the chance of nuns developing ovarian and endometrial cancers would plunge by up to 60 per cent if the church relaxed its opposition to contraception for them.

"If the Catholic church could make the oral contraceptive pill freely available to all its nuns, it would reduce the risk of those accursed pests, cancer of the breast, ovary, and uterus, and give nuns' plight the recognition it deserves," they wrote in an article published by The Lancet medical journal today.

But a senior Australian bishop says there is nothing stopping nuns from taking the pill for medical reasons.

"There's no ethical issue in this at all," said Father Brian Lucas, general secretary of the Australian Catholic Bishops' Conference.

"This would be a matter between a nun and her doctor.

"If she needs hormonal medication, and it could be for any number of things, there's no problem with any woman going to the doctor for those purposes."

Dr Britt, of Melbourne's Monash University, said she feared nuns were getting a mixed message about the pill from the church, given its well-known opposition to all contraceptives.

She hoped more nuns would take the pill to protect their health.

"Fifty years on from when the pill first became available we can say it is good for you and has a big reduction in your chance of developing uterine and ovarian cancer," Dr Britt said.

Two major studies of the health effects of the pill in the United States found it cut cancer deaths by 12 per cent and reduced the risk of ovarian and endometrial cancers by 50 to 60 per cent.

In their article, Dr Britt and Prof Short said nuns and other women who don't have children had a higher risk of ovarian and uterine cancers because they had more menstrual cycles than those who gave birth.

But they said women taking the pill should be aware of its side effects, including blood clots.

Pope Paul VI spelled out the Catholic church's opposition to contraception in Humanae Vitae in 1968, not long after the pill first went on sale.

However, the document notes that "the church in no way regards as unlawful therapeutic means considered necessary to cure organic diseases, even though they also have a contraceptive effect".

In 2001, Spanish catholic nuns were given permission to take the pill if they lived in war zones and faced the threat of rape.