Sonodynamic therapy is a proposed form of treatment using drugs that only become cytotoxic upon exposure to ultrasound. Since ultrasound can be focused into small tissue volumes within the body, this method provides a potential means of localizing treatment and reducing the risk of toxic side-effects elsewhere in the body. In this respect it is similar to photodynamic therapy, which uses light for drug activation, and there are several drugs that have been shown to be sensitive to both light and sound. The main advantage of sonodynamic over photodynamic therapy is the much greater tissue depth that can be reached non-invasively by ultrasound compared to light.