If you take insulin, chances are, you use a strength of insulin called U-100 insulin (strength is not the same thing as type). U-100 insulin is the most common strength of insulin in the US. The U-100 means that there are 100 units of insulin per milliliter of liquid. If you’ve traveled abroad, you may have used or seen another strength of insulin called U-40 insulin. As the name implies, this insulin has 40 units of insulin per milliliter of liquid. U-40 insulin is not used in this country. And there’s yet another strength of insulin, U-500, that is used in cases of insulin resistance.