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Doctors have warned patients about drug interactions with grapefruit juice for years, but it turns out the tart drink might actually interact with certain drugs in a good way, reducing dosages and, therefore, costs and side effects, a new study shows.

Researchers at the University of Chicago medicine wanted to test sirolimus, a transplant drug, on cancer patients, but they knew only small amounts of the drug - 14 percent - is absorbed into the blood stream. Knowing higher doses can cause negative side effects like nausea and diarrhea, they went about searching for a supplement that would boost sirolimus absorption, lead researcher Dr. Ezra Cohen told ABCNews.com.

That was when Cohen said he remembered grapefruit juice can increase blood levels of certain drugs. In fact, the FDA warns that the juice can cause overdoses when combined with anything from cholesterol medication to antihistamines.

"We saw that not as a problem but as an opportunity to enhance the pharmacology to not only sirolimus but to a wide range of drugs," Cohen said.

So they gave some patients grapefruit juice to get more sirolimus into their bloodstreams.

It worked, increasing sirolimus levels by 350 percent and lowering the necessary doses from 90 mg per week to between 25 and 35 mg per week.

"We're talking about cutting those costs by a half to a third," Cohen said.

But things weren't always so rosy. At first, Cohen gave his patients grapefruit juice, but nothing happened. That was when the Florida Department of Citrus got wind of the study and offered to test a sample of the grapefruit juice Cohen's team was using.

"Dr. Cohen didn't realize that the compound [that enhances drug absorption] can be degraded kind of drastically," said Dan King, the director of scientific research at the department. "This juice he was using didn't have a whole lot of this compound present."

The compound is furanocoumarin, and it works by inhibiting enzymes in the intestine that would otherwise limit drug absorption, King said. Cohen's juice had almost no furanocoumarin because it was purchased from the non-refrigerated section of the grocery store. Non-refrigerated juice has to be heated to temperatures that degrade the farancocoumarin, but they wouldn't taste any different from the refrigerated variety, King said.

The Department of Citrus supplied "potent" grapefruit juice for the rest of the study.

"Sure enough, what they sent was very potent," Cohen said. "It allowed us to reduce the dose of sirolimus dramatically."

Dr. Jerry Avorn, chief of the Division of Pharmacoepidemiology and Pharmacoeconomics at Brigham and Women's Hospital in Boston, did not work with researchers on the study, but said he is excited about the results.

"The cost of cancer treatments are very problematic for a lot of patients," Avorn, who also teaches at Harvard Medical School, told ABCNews.com. "If this were to pan out in subsequent studies, this could really be a boon in terms of affordability of these drugs."

Unfortunately, the study did not show that sirolimus was effective against cancer, but Cohen said the results were expected because they weren't using sirolimus with other treatments, and it is known to be less effective alone.

"It's important not to see this as a new cure for cancer," Avorn said. "But rather, it's a very interesting way of using a known food-drug interaction as a means of getting better drug levels into cancer patients."