The drugs can act like chemotherapy in that the AI's suppress estrogen. They are prescribed to those whose cancer cells are 92% or more estrogen. No estrogen, cancer cells die. Therefore, the AI's only target cancer cells. Chemotherapy drugs do not discriminate between good cells and cancer cells and kill both. Hence all the side effects. The reason we get all the side effects with the AI's is because so many aspects of our body make use of estrogen. While the drugs aren't indiscriminately killing cells, they are still affecting them. Think of menopause on steroids.