Regular, long-term use of melatonin has been associated with an increase in fractures, which is also the case with some prescription sleep medications. This is despite the fact that melatonin may slightly increase bone density. For more details and an explanation of this apparent paradox, see the "Concerns and Cautions" section of the Melatonin Supplements Review.
It may be also be advisable to avoid using melatonin, particularly at high dose, after a fracture, as proper fracture healing may be impaired, as also noted in the Review.
Also see answers to the following questions:
What is the best way to take melatonin to get a good night's sleep?
Is it safe to take melatonin when taking prescription sedatives or sleep aids like lorazepam (Ativan) or zolpidem (Ambien)?
Which supplements can help me sleep?
I have been having dizziness for the past few months and am wondering if it could be a side effect of supplements I take. Which supplements cause dizziness?
Are there any supplements I should avoid when taking Valium or Xanax?
Do any supplements help prevent or treat osteoporosis?