Untreated or poorly treated diabetes especially Type 2 has well known complications some of which are discussed below. They can be reduced by adequate control of the condition, regular check-up and prompt treatment of any complications as they arise.
1. Sudden Collapse, Coma
The most common complication of Diabetes is sudden collapse with or without coma due to very low blood sugar. This could arise if a diabetic failed to eat after taking insulin injection or exercised vigorously immediately after taking insulin. Excess insulin can also cause it.
2. Damage to the eyes:
After about fifteen (15) years, about two (2) percent of diabetics may become blind while about ten (10) percent may become partially blind, and about 75 percent, less degree of damage. Cataract is also a known complication.
These complications can be prevented by regular eye checks and proper treatment.
3. Damage to the kidneys:
This could lead to renal failure. About 15 percent of those with type diabetes 2 could develop end-stage chronic kidney disease if the diabetes is not properly managed. The situation is worse with type 1 diabetes where the percentage gets up to 40.

Source: Curled From Possible Complications of Diabetes