More than half of American adults have looked online for health information. Many -- especially those with serious illnesses -- find a community of fellow patients on social media sites.

Now, health researchers are using those posts to learn about why people do or don't stick to their medications.

A team of University of Pennsylvania researchers designed a web crawler to comb more than a million posts over an eight-year period. Their goal: To see what people were saying about aromatase inhibitors, a class of drug used to prevent the recurrence of breast cancer.

Previous research shows that as many as a third of women prescribed aromatase inhibitors stop taking them before their doctors advise, largely due to side effects.

The Penn researchers found that of those who talked about the drug, almost one in five talked about side effects, mostly severe joint pain. Forty percent talked about discontinuing or switching drugs.

Lead author Dr. Jun Mao said doctors need to pay attention to this kind of chatter.

"I think a take-home point is for physicians to ask their patients, 'What are you reading online? What are you concerned about?' And then (provide) helpful information for patients," Mao said.

Mao, an assistant professor of family medicine and community health at Penn's Perelman School of Medicine, said tapping into online forums can provide information about patient experiences with side effects much more quickly than using focus groups or more traditional methods.

"I think there's a tremendous amount of information we can learn online, and it will help inform developing interventions to improve clinical care," Mao said.

He argued the health system is trailing the business world in tapping into data from social media sites to help guide their practices.

Mao is not alone in using interactive websites for medical research. At the Mayo Clinic, Dr. Marysia Tweet has used social media to recruit participants for a study on a rare type of heart disease.

"There are a lot of things to iron out in regards to how to do this, keeping in mind patient data confidentially and security and that kind of thing," Tweet said. "But I do think it's a wonderful resource that we need to learn more about because it will help propel research efforts into the future."