New findings add to mounting evidence that long-term use of a certain type of heartburn medication called proton pump inhibitors could raise the risk of hip fractures in post-menopausal women, especially for those who smoke.

Nearly 10 million Americans experience heartburn, many of whom are on proton pump inhibitors, such as Prilosec or Prevacid, to manage their symptoms.

Former and current female smokers who reported taking proton pump inhibitors at least three times a week for more than two years had a 50 percent increased risk of getting a hip fracture, according to findings published Tuesday in the British Medical Journal.

Harvard researchers analyzed data from nearly 80,000 women enrolled in the Nurse's Health Study, which has tracked the health and lifestyle of thousands of nurses through questionnaires since 1982.

The use of proton pump inhibitors has skyrocketed since 2003, when the class of drugs was made available over the counter. Prescriptions for the class of medications jumped 28 percent, from an estimated 74 million prescriptions in 2002  to 95 million prescriptions in 2009. Proton pump inhibitors became the third-largest class of drugs in the country - with $13.6 billion in sales in 2009 - according to IMS Health, a health-care market research firm.

In May 2010, the U.S. Food and Drug Administration issued a warning about the risk of fractures after a series of studies suggested a possible association. 

It's unclear exactly how proton pump inhibitors are connected to hip fractures. Some experts theorize that the medication can block calcium absorption in the stomach or perhaps deplete the body of vitamin B12, both of which can increase bone frailty. 

The study found that women could reverse their risk by stopping the medication for two years.

Switching to other medications such as antacids or calcium supplements may work just as well to reduce heartburn symptoms for some, said  Dr. Andrew Chan, associate professor of medicine at Harvard Medical School, and co-author of the study.

"What we try to do more often now is review the indication for being on PPIs  on an individual basis," said Dr. Hamed Khalili, a gastroenterologist at Massachusetts General Hospital and lead author of the study." There are few reasons to be on this medication for too long."

Still, for many, proton pump inhibitors are the first-line treatment for heartburn. And experts said those worried about the risks should talk to their doctors before they stop taking the medication.

"Sometimes the benefits of the medications outweigh the outcome," said Chan.