Kadcyla - or T-DM1 - is given to patients who have failed on conventional treatment with Herceptin and chemotherapy.
It is only suitable for patients with the defective Her2 gene and trial results have shown it can extend life by six months compared with two other drugs, lapatinib (Tyverb) and capecitabine (a type of chemotherapy).
Kadcyla is designed to penetrate cancer cells and destroy them from within and, because its action is so precise, a normally toxic form of chemotherapy can be used. Clinical trial results have shown that women on Kadcyla survived 30.9 months compared with 25.1 months for patients treated with lapatinib and capecitabine.
They also suffered fewer side-effects such as diarrhoea and vomiting.
James Jopling, director for Scotland at Breakthrough Breast Cancer, said: "Kadcyla has more manageable side effects and gives a better quality of life than that provided by existing treatments, which could make a huge difference."
Kadcyla is an antibody-drug conjugate which means it uses a "stable linker" to join Herceptin and a powerful chemotherapy together.
This enables a two-stage attack on cancer cells. Kadcyla blocks the growth signals that enable cancer cells to survive before releasing chemotherapy directly into the cells. It is administered intravenously once every three weeks.
The drug has not been appraised for use on the NHS by the Scottish Medicines Consortium.
Mr Jopling said: "Women with secondary breast cancer deserve the extra months that medicines such as Kadcyla offer."