The much-vaunted breast cancer drug Herceptin carries a greater risk of heart damage than previously thought, research indicates today.

The first study looking at use of the drug outside clinical trials has found that 28% of patients with advanced breast cancer suffered cardiac problems. Previous evidence from clinical trials found that between 10% and 26% of patients experienced cardiac problems, depending on its combination with other drugs. One frequently cited US study found that 18.6% of women had to stop taking the drug because of cardiac problems.

The new research, by scientists at the University of Texas, indicates that the risk is higher. Publishing their research in the Journal of Clinical Oncology, the authors describe the risk as "acceptable", and say they found that the majority of the heart damage could be reversed with treatment.

The US Food and Drug Administration warned last year that Herceptin could result in congestive heart failure, leading to inability to pump enough blood throughout the body, or a dysfunction in the heart's ventricle chamber.

The latest study followed 173 patients with advanced stage breast cancer who used the drug for at least a year.Forty-nine (28%) were found to have experienced a "cardiac event" after a follow-up of 32 months, of whom 46 experienced cardiac toxicity "potentially associated with heart failure", and three patients experienced some decrease in ventricle function. The majority (31) experienced the problems while being treated with Herceptin alone; the other 18 were being treated with a combination of Herceptin and chemotherapy. There was one cardiac-related death. When patients stopped using Herceptin and were treated with heart drugs such as beta blockers, all but three had improved heart function. And, after repairing the damage, patients could resume Herceptin treatment.

The authors, led by Francisco J Esteva, said: "The drug substantially prolongs survival ... If the cardiac side-effects of Herceptin treatment can be managed, the drug is safe to use."

John Toy, Cancer Research UK's medical director, described the study as reassuring since it demonstrated for the first time that Herceptin could be restarted once the women affected had been treated. But he added that the National Institute for Health and Clinical Excellence, which has authorised use of the drug for early and advanced breast cancer, stressed when approving the drug that heart function must be assessed beforehand. It also said the drug should not be given to those at risk of heart failure and patients should undergo regular assessments.