"It's a drug that was discovered in the 70s," Dr. Ezra Cohen, a cancer specialist at the University of Chicago Medicine and the study's lead author, told FoxNews.com.  "It was clearly shown to have anti-cancer effects and anti-neoplastic effects, but it hadn't been developed for cancer extensively because the patent ran out.  There wasn't a lot of commercial interest to develop sirolimus, so it sort of was pushed aside for a while.  Eventually, sirolimus was indeed approved, but for people who got organ transplants to prevent rejection."

Cohen and his team study how food consumption can either help or hurt the effectiveness of drugs used for cancer therapy.  They got the idea to test grapefruit juice from another previous study around two decades ago.

"Investigators were doing a study on alcohol's effect on a certain heart drug," Cohen said. "And to mask the taste of alcohol, they used grapefruit juice.  What they ended up finding out was that the grapefruit juice increased the blood levels of the drug."