It is extremely rare for someone to be what is called a brittle diabetic...that is one who's body cannot use the insulin that is injected or the pill...
In my years nursing I had only seen 1 brittle diabetic and his blood/sugar would sometimes go over 1000....it did much damage to his body, but there was nothing at the time that could be done for such people...
I am just guessing but I think at 15 it will probably be insulin...the injections are what is called sub Q, and are not painful. Not like getting a shot of penicillin with those big needles...
2 of my 4 sons has mature onset of diabetes, they both take pills, but if they lost some weight, it might help...diet is important in a diabetic. Your family will need lots of teaching on the subject. Don't be afraid to ask questions. If she is in the hospital, she will probably be seen by a dietitian and specialist in endocrinology..
hope this helps. Diabetes is not cured, its controlled..
goat granny is senile, she though you were talking about a 15 year old person, after reading replies, I gather its a 15 year old cat....don’t know anything about cat diabetes...Signed silly goat granny