Some breast cells divide and grow when a protein (human epidermal growth factor) attaches itself to another protein called HER2. Herceptin blocks this process by attaching itself to the HER2 protein, so that the epidermal growth factor cannot reach the breast cancer cells.

Herceptin only works in people who have high levels of the HER2 protein, which you can be tested for. About one in five breast cancers in women are sensitive to Herceptin, but it's thought that fewer men have tumours that are sensitive to it.

Herceptin can be used to treat early breast cancer or breast cancer which has spread (secondary breast cancer).