A growing number of teenage girls in Canada use oral and other forms of hormonal contraceptives. Doctors prescribe them for birth control and other reasons. A review published in January in the journal Clinical Endocrinology has identified a serious long-term risk: a strong link between the use of combined estrogen and progesterone and loss of bone mineral density.

Researchers at the Centre for Menstrual Cycle and Ovulation Research at the University of British Columbia looked at published studies that compared women age 12 to 19 who used combined hormone contraceptives and compared them to adolescents who did not take hormone treatment. They compared how much accrual of bone mineral density took place during a 12 to 24 month period. A total of 1,535 adolescents were compared over a 12-month period and 885 adolescents over a 24-month period.

Throughout both time intervals, teenage girls on hormone treatment had statistically and clinically significantly lower bone mineral density than those not on treatment.

This is the first meta-analysis showing changes in bone mineral density in the spines of otherwise healthy adolescent women followed over time. The authors concluded that hormonal contraceptive treatment in adolescence could impair peak bone mineral density in the spine at a time when teenage girls are supposed to be acquiring their peak bone mass.

Dr. Jerilynn Prior is the scientific director of Centre for Menstrual Cycle and Ovulation Research at the University of British Columbia. (University of British Columbia)

In order for the bones to reach peak bone mineral density, a process called bone modeling must occur. Other studies have shown that combined hormone use that includes an estrogen called ethinyl estradiol at doses from 20 to 35 micrograms suppresses bone formation in young women. The authors hypothesize that combined hormone use during the teen years suppresses bone modeling and impairs bone mineral density accrual during the critical period when peak bone mass is supposed to be achieved.

Achieving the highest possible peak bone mass helps prevent collapsed vertebrae and other fractures later in life; suppressing that process does the opposite.

And there is already evidence of harm. Several large prospective studies (from Canada, the United States, Brazil and China) showed an increase in fractures in women who used combined estrogen and progesterone hormonal treatment.

These studies come at a time when doctors in Canada are prescribing combined hormone treatment more often to adolescent women. Once given mainly for birth control, doctors prescribe them for other reasons. For instance, they prescribe them to relieve menstrual cramps and to treat irregular periods. They also prescribe them commonly to treat acne.

Today, a higher percentage of women than ever take combined hormones in adolescence rather than wait until early adulthood. According to the Canadian Multicentre Osteoporosis Study, 65 per cent of women ages 16 to 19 are taking or have taken combined hormone treatment. It's the increased use in young women coupled with the potential for osteoporosis and fractures that led the authors of the meta-analysis to call the findings a potential public health problem.

Other concerns have been raised about the acne drug Diane-35, which contains the hormones cyproterone and ethinyl estradiol. In 2013, France withdrew sales of Diane-35 in response to the deaths of four women over a 25-year period. Subsequently, a committee of the European Medicines Agency concluded that the benefits of Diane-35 and its generics outweigh the risks when measures are taken to minimize the risk of blood clots. At the time, Health Canada said that the benefits outweighed the risks. The Canadian regulator also said Diane-35 should not be used in patients with a medical history that puts them at risk for blood clots, including smoking, being overweight or having a family history of clots.

The Society of Obstetricians and Gynaecologists of Canada concluded the risk of thromboembolism in Diane-35 users is very low and comparable to that of other combined hormone contraceptives.

At the time, no concerns were raised about the risk to bone mineral density in teenage users of hormones.

Dr. Jerilynn Prior and her co-authors of the current meta-analysis say their findings can only show an association between the use of combine hormone treatment and decreased bone mineral density. They called for placebo-controlled trials pitting users of hormones against non-users to see if the results hold up.

Meanwhile, they say there are safer ways to ensure effective teen birth control and the treatment of cramps, irregular periods and acne. They say adolescent women and their parents need to ask for other kinds of treatments first. They add that family doctors and nurse practitioners need to know about the risk advise their teen patients accordingly.