Hello lybrate-user. The difference between 40 and 100 IU is the insulin concentration per ml. The higher the number (e.g. 300IU) the higher the concentration. Ie. 1 ml of 40 IU contains 40U, 100IU contains 100U and 300 contains 300U. All you need to do is make sure your insulin delivery
system is compatible. You get different syringes for 40 and 100IU insulin. Hope this helps.