Birth control pills can protect women against ovarian cancer for 30 years or longer after they stop taking them and have so far prevented 100,000 ovarian cancer deaths worldwide, British researchers said on Thursday.

The longer women stay on the pill, the lower their risk of developing the disease, which is more common after age 50, the researchers wrote in the journal Lancet. For example, women who take the pill for 15 years cut their risk in half, they said.

Worldwide the pill has already prevented 200,000 women from developing cancer of the ovary and has prevented 100,000 deaths from the disease, Valerie Beral of the University of Oxford and colleagues wrote in their report.

The findings are the strongest evidence yet of the benefits of the pill when it comes to ovarian cancer, and show the protection lasts far longer than people had thought, Beral said.

“When you are 60 it matters whether you took it for five years or 10 years in your twenties,” Beral said in a telephone interview. “The longer you took it, the better off you are when the risk of ovarian cancer is high.”

An estimated 300 million women have used the contraceptive pill since its introduction in the early 1960s. Hundreds of studies have looked at its safety, some suggesting benefits and others showing a raised risk of breast and cervical cancer.

Beral and colleagues said their research, analysing 45 studies on ovarian cancer in 21 countries, shows that the benefits of the pill outweigh the risks. Ovarian cancer is particularly deadly because women often have mild or no symptoms until the disease has progressed.

The breast cancer risk - which also extends to stroke and blood clots - is much smaller and exists only while women are taking the pill and soon after they stop, Beral added.

“Whereas for ovarian cancer the protection persists for decades,” Beral said.

Taking the pill for 10 years cut the risk of ovarian cancer before the age of 75 from 12 per 1,000 women to 8 per 1,000. It also reduced the risk of dying from the disease from 7 per 1,000 women to 5 per 1,000 before the age of 75, the study found.

More than 100 million women now take the pill, so it will eventually prevent more than 30,000 ovarian cancer cases annually over the next few decades, the researchers wrote.

The study also showed ethnicity, education, family history and other factors do not seem to make much difference in reducing risk when it comes to using the pill.

The researchers said they did not know exactly why the pill provides protection but said the benefits make sense because the drug suppresses the ovaries’ function when women are taking it.

Worldwide there are more than 190,000 new cases of ovarian cancer a year, the International Agency for Research on Cancer says.