Just days before nurse Barbara Clark made the headlines by convincing her local NHS trust to supply the drug, the manufacturer had revealed to America's drugs regulator, the Food and Drug Administration (FDA), that it "significantly increased" the risk of heart problems.
So far, the drug has killed 15 women in the US, where it has been available for several years, and there have been 62 reports of serious side-effects. This does not include the flu-like symptoms that afflict nearly half of all users.
Campaign groups that have been lobbying for the drug to be made available on the NHS claim that it may save 1000 lives a year - but, according to the latest safety figures, it could kill 1750 women.
Every woman in the UK with early-stage breast cancer is to be tested to see if Herceptin therapy is appropriate. The drug, given as a saline drip, counteracts the activity of a protein called 'HER-2', the cause of the breast cancer in around one in seven cases.
It is not known if they will follow the lead of clinicians in America who, aware of the drug's serious side-effects, are also testing the health of women's hearts and lungs before prescribing the drug.