Like "studies have been done and they say anything over 150 for a long period of time can be hurtfull long term. As a rule of thumb, a spike to just under 200 for a T2 is normal. Over 200 shows that there is an issue, be it medical, what the person ate, or meds, taken or not taken. A T1 may spike and come down more rapidly. But somone who does not take care of them selves and keeps a constant BS over 200 is sure to expirence issues. I didnt take care of myself for a number of years. When I finally got back under a doctors care, I had an A1C of 12.9 which they say is an average of 240. My eye doctor told me that I had started to break some of the capilaries in the back of my eyes. If I continued, it would have led to blindness. So, rule of my thumb, keep normal as much as possible, keep spikes to under 200. Live long and prosper. An article about this:"