LA JOLLA, Calif. -- Some local scientists believe they have found a way to turn painkillers into cancer killers.

For years, scientists have long observed that people who regularly take painkillers, such as aspirin or ibuprofen, had lower incidences of certain types of cancer. However, scientists never knew exactly why.

Scientists at San Diego's Sanford-Burnham Medical Research Institute believe they now know that answer.

In a study published recently in the medical journal Cancer Cell, scientists revealed how one painkiller called Sulindac shuts down the growth of certain cancer cells.

"[It stops] colon cancers, breast cancers, liver cancers and even prostate cancer and it seems pretty powerful," said Dr. Xiao-Kun Zhang.

Sulindac is currently prescribed for the treatment of pain, fever and relief of arthritis symptoms.

The problem is Sulindac and other painkillers are also known for their potentially dangerous cardiovascular side effects. After identifying what kills cancer cells, researchers at Sanford-Burnham tweaked Sulindac and created a new version of the drug, called K-80003.

A tumor shrank with Sulindac, and with the new drug, it further reduced in size. Scientists said it took three weeks for the tumor to shrink to less than half of its original size.

The negative effects of taking the painkiller were also reduced. Scientists said it was the most encouraging discovery researchers have made in the experiment, which has been going on for more than 20 years.

"If we can reduce the side effect, this is a big, big deal," said Zhang.

The findings may help drug makers develop new drugs to fight cancer.

Tests have so far been done using animals. Clinical tests on humans may start in the next year.