U-100 insulin syringes have orange caps and black scale while U-40 syringes have red caps and red scale. U-100 indicates the concentration of insulin. U-100 means there are 100 units of insulin in every 1 ml. A U-100 syringe must be used for insulin of U-100 strength while a U-40 syringe must be used for U-40 insulin strength. If someone uses U-100 insulin with U-40 syringes, there is a risk of getting 2.5 times as much insulin as expected. However all insulin pens throughout the world are U-100.