The study team was led by Margaret Urban at the National Health Laboratory Service (NHLS) in Johannesburg and Professor Emily Banks of The Australian National University. Professor Banks said the study builds on previous research that found that women using oral contraception have a temporarily increased risk of breast cancer and cervical cancer and a decreased risk of ovarian cancer and cancer of the lining of the womb.
"Our study confirms previous findings that the use of oral hormonal contraceptives is associated with an increased risk of breast and cervical cancer," said Professor Banks.
"We have also shown for the first time that injectable hormonal contraception use independently increases the risk of developing these cancers to a similar extent as oral contraceptives, and that these risks wear off over time."
The researchers found that South African women who had recently used one or both of these contraception methods were about 1.7 times more likely to develop breast cancer and 1.4 times more likely to develop cervical cancer than women who had never used hormonal contraception.
However, this increased risk disappeared within a few years after women stopped taking the contraception.
The study also supports previous findings that extended use of hormonal contraception is associated with a reduced risk of ovarian cancer and cancer of the lining of the womb.
"Over 60 million women are using injectable contraceptives world wide. These findings fill an important gap in knowledge, which will inform women’s contraception choices," said Urban.
"The temporary increase in the risk of breast and cervical cancer and the long term protection against ovarian and endometrial cancers need to be considered alongside the highly effective contraception that these medications provide," said Urban.
The study, involving more than 3,500 black women, was run as part of the Johannesburg Cancer Case Control Study at Johannesburg public hospitals and was chiefly funded by the NHLS and the South African Medical Research Council. Among black South African women, injectable contraceptives are used more commonly than the oral contraceptive pill.
Oral contraceptive pills contain man-made versions of the female sex hormones estrogen and progesterone, while injectable contraceptives usually only contain progesterone.
