But with Olaparib, scientists in Britain have found a way of exploiting a fault inherent in BRCA mutation cancer cells to destroy them.
"By giving this drug we have made what has been an advantage to the cell in fact an achilles heel," said Dr Johann de Bono, of the ICR.
"This is really the holy grail of cancer treatment - selectively killing cancer cells and sparing normal cells."
Traditional cancer treatments, like chemotherapy, kill both healthy and cancerous cells.
They destroy more of the cancer cells because they grow faster, but produce terrible side effects like hair loss, nausea and fatigue.
In contrast, Olaparib leaves healthy cells untouched. It is highly potent with only mild side effects.
In the first clinical trial nearly two thirds of patients with the BRCA mutation responded to the drug.