Cancer chemotherapy can have widely different side effects depending on what time of day it is administered, new research shows.

"We got much more drug in with much less toxicity" by controlling the time of delivery, says researcher William Hrushesky of the University of Minnesota Medical School.

His work is based on the science of chronobiology, which takes into account chemistry changes controlled by the body's internal clock.

Hrushesky's study involved 31 women with ovarian cancer being treated with two cancer drugs. One group was given adriamycin at about 6 a.m. and cisplatin at 6 p.m.; the other group received the drugs at the opposite times.

Those in the first group had fewer infections and less bleeding -- meaning fewer interruptions of therapy -- than those in the other group. "It was all simply because of the timing," Hrushesky says. "Everything else was exactly the same."

There was no discernible difference between the two groups in the effect of the drug on the cancer, the study found, but both groups had better response than patients not receiving the drugs 12 hours apart.

"Medical practice totally ignores when any drug is given," says Hrushesky. But with cancer chemotherapy, there is a fine line between its curative effects and its toxic effects. By choosing the right time of day, the chemicals seem to become more "selective" -- able to differentiate between healthy cells and cancer cells.

"And any edge we can get has got to be of some benefit," he says.