All they need is reassurance that their fertility will return with reduced physical stress. There is no need to take drugs to bring it back. The idea that cycles or menstruation can be regulated by contraceptive medication is pure mythology. The medication abolishes the normal cycle and replaces it by uterine bleeding which is manipulated by the chemicals administered. The most important way for women to protect themselves from osteoporosis in later life is to maintain an active exercise programme when they are younger.
Exercise is the most important influence of all in maintaining good bone structure.
So the effect of exercise would protect against any effect on the bones resulting from perhaps several months without ovulatory cycles. However, it would be prudent for these young women to so organise their competative schedules that they regularly have long times without sufficiently intense physical training to cause amenorrhea. The classical study was carried out at Harvard University in USA by our colleague Prof. James Brown and some others working together. The subjects of the study were women rowers training for the Olympics. This was about ten years ago.