Injectable birth control has undeniable benefits. You don’t have to remember to take your pill every day, and it’s very reliable. For women who work shifts, or have unpredictable schedules, that can be a bonus. But there’s also a downside.

Some women who take the birth control shot (full name medroxyprogesterone, brand name Depo-Provera) find it weakens their bones. The shot has been linked to a loss of bone strength, which is gauged by measuring bone mineral density. Weaker bones are more likely to break.

A recent study from Galveston, Texas, found that half of the women who had medroxyprogesterone shots lost at least 5 percent of their bone mineral density during two years of treatment. That’s a significant amount, and could be hard to make up afterward, depending on your age. Most women’s bone strength peaks at about age 30, then declines afterward.

But not everyone was affected. Researchers tried to tease out which factors made it more likely that women taking the shot would lose bone density. They found that smoking, eating little calcium, and not having had a child already, were all linked to losing bone strength.