A significant number of patients with advanced breast cancer using Herceptin face the risk of heart damage, research has indicated.
The US study links Herceptin to heart damage
US scientists followed 173 patients taking the drug and found 28% developed some form damage to their hearts.
But the team, writing in the Journal of Clinical Oncology, said this was an "acceptable risk" as the damage could be repaired in the majority of cases.
Currently, Herceptin is not recommended for patients with heart problems.
The advice is based on clinical trials that revealed a link between cardiac damage and Herceptin use.
But scientists, based at the University of Texas MD Anderson Center, said the risks had not been quantified.
The researchers looked at 173 patients with advanced breast cancer who had been taking Herceptin, also known as Trastuzumab, for one year.
Each patient was given a cardiac assessment and they were then followed for 32 months.
The scientists found 49 of the patients (28%) had experienced some form of damage to their heart - the majority suffering damage linked to heart failure, in one case leading to death.
Thirty-one of these patients suffered the damage while taking Herceptin alone, the other 18 while taking Herceptin combined with chemotherapy treatment.
For 46 of the 49 patients affected, heart function improved after halting Herceptin treatment and taking conventional heart drugs such as beta-blockers or ACE inhibitors, the scientists found.
And the researchers said Herceptin treatment could be resumed after the damage was repaired.
Lead author Francisco Esteva, an associate professor at the MD Anderson Cancer Center, said: "The drug substantially prolongs survival and while we found substantial cardiac toxicity, we also discovered that this side-effect can be successfully treated, which was not clearly known before this study.
"If the cardiac side-effects of Herceptin treatment can be managed, the drug is safe to use."
He said patients with advanced breast cancer should have a heart assessment before and during treatment.
The study did not look at patients with early-stage breast cancer.
The National Institute for Health and Clinical Excellence (NICE) gave its initial backing for the NHS in England and Wales to use Herceptin for early-stage breast cancer in June. In Scotland, Herceptin is already available on the NHS.
NICE's final decision is pending the outcome of an appeal by the Newbury and Community Primary Care Trust, which says the NHS drugs watchdog's guidelines are not clear on patient treatment, cost effectiveness and potential risks.
Professor John Toy, Cancer Research UK's medical director, said: "It has been known for some time that Herceptin can cause heart problems for some women.
"Current NICE guidelines outline that, before using the drug, all women should receive cardiac assessment and be monitored throughout their treatment to minimise the risk.
"This reassuring study shows that, by stopping the drug in the women who have an adverse reaction and successfully treating the heart problem, the Herceptin treatment can safely restart."
Dr Sarah Rawlings, head of policy and information at Breakthrough Breast Cancer, said: "All drugs have benefits and side effects, which is why there are different treatments available to suit individual women.
"Anyone with concerns about their treatment should speak to their doctor."