There are different types of cinnamon; in the UK we normally have ceylon cinnamon but the cinnamon shown to have greatest effect on BG levels was a cassis cinnamon.
A study published in the journal Diabetes Care in 2003 looked at 60 men and women with Type 2 diabetes who were taking diabetes pills. The participants took either 1, 3, or 6 grams of cassia cinnamon or a placebo, in capsule form, for 40 days. After this time, blood glucose levels dropped between 18% and 29% in all three groups that received cinnamon. However, only the participants who had taken the smallest amount of cinnamon (1 gram) continued to have improved blood glucose levels 20 days after they stopped taking it, for reasons the researchers didn’t quite understand.
A word of caution, too much cinnamon acts as a laxative (I was unfortunate enough to find that out myself)