It is necessary to control rodents. One of the main ingredients is the same chemical used as a blood thinner in humans to help prevent strokes. In a dose which would not harm a person it will cause internal bleeding which will cause the animals death.
As with many household chemicals, used as directed and in appropriate places, they can be safely used.
Please note I have 4 cats. Each fall I still have a few mice who try to move in. The cats usually discover them before I do. One is a particularly good mouser. I rescued her from my friends lake home where she had had to fend for herself. She was not their cat and was very thin. She is a very sweet and loving cat.
