Is ketosis considered dangerous?

Yes: It depends on the degree and duration. In diabetics it can be life threatening. It lowers the acidity of your blood. People on fade diets like atkins or starving themselves also go into ketosis, but the body can manage to regulate it for a while.

No! Only in Extreme: When we burn fat, body fat or fat from foods, we produce a variety of ketone bodies. While physicians have often associated this state with unusual unhealthy conditions (usual Doc focus), staying in a chronic state of mild ketosis is quite healthy; strongly associated with healthy lipoprotein particle concentrations, low rates of diabetes, obesity, cardiovascular disease, cancer & mortality.