Medical company Genentech says it has stopped enrolling ovarian cancer patients in a test for Avastin, because a higher-than-expected number of the women developed perforations in their gastrointestinal tracts.

Genentech says five of 44 patients involved in the study, 11 percent, have developed perforations as a result of taking the drug.

In previous tests, similar results were shown, however they were present in a much smaller percentage of patients.

Nelson Teng, chief of gynecologic oncology at Stanford Medical School, has been involved in tests and says, "this must be an added side effect of Avastin," adding, "it's a little perplexing."

Avastin is Genentech's second biggest-selling drug, and it has been approved for use against colon cancer and tested in breast and lung cancer.