Being chronically underweight or having a slight frame. Having osteoporosis in the family. A poor diet, low in vitamins and minerals, especially calcium. Being sedentary and lack of weight-bearing exercise. In women this lowers the estrogen content of the blood, thus weakening the bones.
Smoking is particularly dangerous for women who have other risk factors for osteoporosis.
Long-term use of certain medications. Some people with asthma and rheumatoid arthritis take cortisone for long periods, which can diminish bone strength. So can long-term use of thyroid hormones, which are sometimes used to treat obesity, although most physicians do not recommend them for this purpose.