Just googled to check that out now.....;)
New study suggests breast cancer treatments lose effectiveness over time
"The current paradigm that we have is that these drugs, over the lifetime of the patient, have a constant effect,” said Dr. Jatoi, chief of surgical oncology at the Health Science Center and the paper’s lead author. “What we’ve suggested in this paper is that’s not really the case. What we’re seeing is that some drugs have an initial effect. The effect of the drug diminishes after about three years.”"
http://www.uthscsa.edu/hscnews/singleformat2.asp?newID=3808