More evidence about the breast cancer drug Herceptin is likely to increase pressure on health trusts to fund the expensive treatment despite the fact that its use among patients outside clinical trials still has to be approved by safety regulators.

A US-led study published today involving more than 3,000 patients in 43 countries suggests the drug reduces the risk of early stage breast cancer returning by up to 51%.

Other data in recent months has suggested similar success rates for the drug, which is licensed in this country for use in advanced breast cancer caused by a gene which makes too much of the protein Her2. Some 20-30% of breast cancer patients fall into this group whose disease takes a particularly aggressive form.

Doctors are free to prescribe it to patients they believe will benefit, even though it has not yet undergone checks for safety or cost-effectiveness. But it costs the NHS £21,800 per patient a year and trusts, many already in financial difficulties, have been reluctant to pay, at least until the drug has gained regulatory approval, which may be next summer.

Patricia Hewitt, the health secretary, recently took the unusual step of instructing managers not to deny Herceptin to patients on cost grounds.

The latest research results involved doctors from the Breast Cancer International Research Group comparing patients given chemotherapy with those whose treatment was supplemented by Herceptin. Dr Andrew Wardley, consultant medical oncologist at the Christie hospital, Manchester, and an advocate of the drug's wider use, said: "These results confirm the outstanding benefit of Herceptin in early breast cancer and make it imperative that all breast cancers are tested for Her2 status at diagnosis and appropriate patients receive Herceptin."