A contraceptive injection could damage your bones, warn scientists.
A single injection of the drug lasts for three months
Researchers say using Depo Provera could increase women's risk of developing osteoporosis after the menopause by adversely affecting their bone density.
But Dr Jennifer Walsh, of the Bone Metabolism Group at the University of Sheffield, said the trends she had discovered so far were only small and said women should not stop using the contraceptive.
"We don't want to be alarmist. The amount of bone loss we are looking at is between 3-4% it is not a big amount.
"But then once these women go through the menopause they could develop osteoporosis a year or two earlier."
Monday has been designated World Osteoporosis Day, part of a global health campaign which aims to raise awareness of the condition.
Dr Walsh is now studying 200 women, half of whom are using Depo Provera, to examine their normal bone development.
Depo-Provera is used by 330,000 women in the UK. It is a popular long-term contraceptive. A single injection of the drug lasts for three months.
It works by releasing progestogen slowly into the body suppressing oestrogen and other hormone levels.
Over the two year study Dr Walsh will also be looking at how the bones in older teenagers and young women develop and what the effect will be in later life if they do not reach their full potential.
Dr Walsh said: "It's becoming more and more recognised that osteoporosis is not just to do with bone loss as you get older, but how much bone you gain during growth and development.
"Bones grow until you are about 30, but if this process is interrupted in some way, an individual may fail to reach their potential peak of bone strength.
"We know quite a lot about childhood bone growth and the need for good nutrition and exercise, but not so much about the factors that control later bone development - such as hormones.
"Contraceptives which alter hormonal activity could have some effect.
"If we find out more about this process and can maximise a woman's bone mass early in life they will be in a better position when they get older and reach the menopause and less at risk of developing osteoporosis."
The study is being funded by the Arthritis Research Campaign and the National Osteoporosis Society.
A spokeswoman for the Arthritis Research Campaign said: "There's a need to know more about why some women are more at risk of developing osteoporosis and we hope this research should shed some interesting new light on this area."
A spokesman for the National Osteoporosis Society agreed more research was needed.
"Several research studies have shown that women on Depo Provera have lower bone density than other women of the same age and some studies demonstrated that bone loss occurred in adolescent girls using this contraceptive (long term users under 21 years had the lowest bone density, especially if they started at a young age).
"However, it appears that bone density improves when a woman stops Depo Provera (before the menopause) and that, in general, the risk of low bone density does not increase the longer it is used."