People with cancer might be able to take less of a chemotherapy drug if they take it with grapefruit juice, a small new study suggests.

Grapefruit juice is known to interact with some types of medications, leading to an overdose hazard.

Researchers at the University of Chicago wanted to take advantage of that effect.

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The potency of grapefruit juice varies. (Mahmud Hams/AFP/Getty)

Dr. Ezra Cohen and his colleagues tested sirolimus, a transplant drug with anti-cancer effects, on 138 people with incurable cancer.

Combining sirolimus with grapefruit juice caused blood levels of the drug to rise to potentially effective levels with relatively few side-effects, the researchers said in Monday’s online issue of Clinical Cancer Research.

Grapefruit juice inhibits enzymes in the intestine that break down sirolimus and several other drugs, including calcium channel blockers used to treat high blood pressure and some cholesterol-lowering statins.

"This is the first cancer study to harness this drug-food interaction," the study's authors noted.

No patients in the study had a complete response, but about a third of patients had stable disease, meaning a period when their cancers did not advance.

The researchers also found that different types of grapefruit juice seemed to vary in potency, depending on the levels of furanocoumarin. The first juice they tried wasn’t strong enough so the investigators ended up using juice supplied by the Florida Department of Citrus.

Individual responses vary

Combining sirolimus with another drug, ketoconazole, resulted in a slightly stronger effect than grapefruit juice.

The advantage is that grapefruit juice itself is nontoxic with no risk of overdose.

The researchers acknowledged challenges in incorporating it into regular practice, such as ensuring it has consistent potency.

The findings are predicable based on what's known about the pharmacology of the drugs involved and grapefruit juice, said Dr. David Juurlink, a scientist at the Institute for Clinical Evaluate Sciences in Toronto and head of the division of clinical pharmacology at the University of Toronto.

"People taking sirolimus should probably avoid grapefruit juice as well as Seville oranges and pomelo, which do the same thing," Juurlink said in an email.

It's premature to suggest that people take lower doses with grapefruit juice since the effect can't be predicted from person-to-person and depends on how much grapefruit juice is taken and for how long, he added.

The study was funded by the U.S. National Institutes of Health.