"I could rationalize and justify taking these pills because doctors prescribed them. It wasn't like I was buying a white bag on the street," he said.

A few years ago, doctors at the Centers for Disease Control and Prevention noticed a trend in deaths from prescription drug use. In 2008, they published a study confirming their observations. Dr. Aron Hall is one of the co-authors.

"We found that a majority of people who overdosed on prescription drugs did not have a prescription for the drugs that killed them," Hall said.

But patients also have access to their own prescriptions, over-the-counter medications and herbal remedies.

"Every medication that you add increases the risk for harm as well," Brand said.

If a patient sees more than one doctor and uses more than one pharmacy, a potentially dangerous consequence is that no one, at least no health care professional, has a complete list of his or her medications. It's also hard to know how someone reacts to even one type of medicine.

"Every patient reacts differently to medication…some are extremely sensitive, some take much, much more medication and don’t have any effect at all. Unfortunately, there’s really no way to know that for sure," Brand said.

Brand recommends sticking with one primary physician, and if you do see specialists, make sure all your doctors know what medicines you are taking. She also recommends using only one pharmacy, and if you want to take something else, ask a pharmacist if it is safe to take with your prescriptions.