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 Researchers analyzed 36 studies including more than 140,000 women
from around the world. They found that every five years of taking
birth control pills was linked to a 24 percent reduction in the risk
for endometrial cancer, even more than three decades after women
stopped using the contraceptives.
“Our results show clearly, for the first time, that the protective
effect of the pill on endometrial cancer lasts for over 30 years,”
senior study author Valerie Beral of Oxford University in the U.K.
said by email.
The most commonly prescribed oral contraceptives contain man-made
versions of the natural female hormones estrogen and progesterone.
Past research has already linked these pills to a reduced risk of
endometrial and ovarian tumors, but also an elevated risk for
breast, cervical and liver malignancies, according to the U.S.
National Cancer Institute (NCI).
Cancer of the endometrium, which is the lining of the uterus,
typically strikes women around age 60, after the end of their
reproductive years, according to NCI. Risk factors include obesity,
never giving birth, starting menstruation at a younger age, entering
menopause at an older age and taking estrogen therapy to ease
menopause symptoms.
Since the first oral contraceptives were introduced in the 1960s,
about 400 million women in high-income countries alone have used the
pills, the researchers note in The Lancet Oncology.
To understand the connection between endometrial cancer and oral
contraceptives, Beral and colleagues examined data on 27,276 women
who got these tumors and another 115,743 who didn’t.
They drew on previous research done in what they described as
high-income countries, including the U.S., Europe, Japan, China,
Australia and South Africa.
Half the women in the analysis were at least 63 years old, and among
those with cancer, half were diagnosed by 2001.
Among the women who did get endometrial tumors, roughly one third
had used birth control pills, typically for around three years.
Almost 40 percent of the women who didn’t get these malignancies
took the pills, and they generally stayed on them for more than four
years.
The risk reduction tied to contraceptives varied by tumor type. Oral
contraceptive use was associated with 31 percent lower lifetime risk
of developing endometrial carcinomas, which are more common. The
pills were also associated with a 17 percent lower risk of less
common sarcomas, which start in the uterine muscle and supportive
tissue.
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In the studied high-income countries, use of birth control pills was
linked to a reduction in the risk of getting endometrial cancer by
age 75. Among pill users, that meant 1.3 women in every 100 would
get uterine cancer, compared with 2.3 women in every 100 who didn’t
use the pill, the researchers estimated.
Over the past 50 years, up to 400,000 out of an estimated total 3.4
million endometrial cancers might have been prevented by oral
contraceptive use in the study countries, the authors conclude.
More research is needed to understand why the pills appear to
protect against cancer long after women stop taking them, the
researchers note.
“Oral contraceptives typically work by preventing ovulation,” said
Dr. Nicolas Wentzensen of the NCI. “We know that factors that reduce
the number of ovulations during a woman’s lifetime are associated
with a reduced risk for both uterine and ovarian cancers.”
Even though the amount of estrogen in birth control pills declined
over the course of the 1960s, 1970s and 1980s, the study didn’t find
differences in risk reduction for endometrial cancer over time,
Wentzensen wrote in an editorial accompanying the study.
“These findings suggest that there is a threshold effect, i.e. a low
level of estrogen is sufficient for the cancer-protective effects
and higher levels do not further increase these effects,” he said by
email.
SOURCE: http://bit.ly/1IkFj7Q and http://bit.ly/1UoQhP0 The Lancet
Oncology, online August 4, 2015.
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