Breast cancer patients are at greater risk of heart damage from the drug Herceptin than previously thought, experts have said.
The drug, approved for use on the NHS in June, caused harm to women with advanced breast cancer in a US study.
The research of patients using it for at least a year found 28 per cent suffered cardiac problems.
But authors said the risk was an "acceptable" one and the majority of heart damage could be reversed.
Herceptin is known to weaken the heart muscle and is not recommended for those with heart trouble.
The National Institute for Health and Clinical Excell ence (Nice) said when approving the drug heart function must be assessed.
It also said it should not be given to those at risk of heart failure and patients should undergo assessments. The drug is licensed in the UK for early and advanced breast cancer.
In clinical trials testing Herceptin with chemotherapy, between ten and 26 per cent of patients were found to experience cardiac problems.
A warning from the US Food and Drug Administration last year said 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.
In the latest small study, not a clinical trial, 28 per cent (49) of patients treated with Herceptin were found to have experienced a "cardiac event" after a follow-up at 32 months.
Of those, 46 patients experienced cardiac toxicity "potentially associated with heart failure", and three patients experienced some decrease in ventricle function.
The majority of these (31) experienced the problems while being treated with Herceptin, after prior combined Herceptin and chemotherapy.
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 then resume Herceptin treatment, the authors said.
Responding to the research, Head of Policy and Information at Breakthrough Breast Cancer Dr Sarah Rawlings, said: "The more data patients have on the effects of drugs like Herceptin the better.
"We know that Herceptin is not suitable for all HER2 positive patients and that it can cause heart problems in some, therefore heart function is assessed before and strictly monitored throughout treatment."