Even though Salvinorin A is not toxic, nothing is known about the long-term side effects of Salvia Divinorum. Experience with another hallucinogen (LSD) teaches us that such substances can and at least sometimes do have serious dangers - such as increased risk of suicide.
At very low dosages, Salvia can act as an antidepressant. Low dosages often yield "interesting" experiences. In larger dosages, the person fully loses control over mind and body, does not percieve anything or anybody like they really are, and can start violently trash about or run out of the house and underneath a car. People who have experienced this generally never want to use Salvia again.