### Why Do Insulin Pens Run Out Faster Than Expected? Many people with diabetes find that their insulin pens don't last as long as they expect. This can be frustrating, but there are a few reasons why this happens. First, let's do some basic math. A typical insulin pen contains 3ml of insulin, and if it's a U-100 pen, that means it has 300 units of insulin (since U-100 means 100 units per ml). If you take 12 units per day, you might expect the pen to last 25 days (300 ÷ 12 = 25). However, this isn't always the case. One reason for this is the need to prime the pen before each use. Priming involves releasing a small amount of insulin to ensure the pen is working correctly and there are no air bubbles. The instructions usually recommend priming with 2 units of insulin, and if you don't see insulin at the tip, you may need to prime again. This can use up to 3 units each time you prime. If you prime once a day, that's an extra 3 units per day, which adds up over time. Another factor is the storage and lifespan of the insulin. For example, Humulin N pens need to be discarded 14 days after first use, regardless of how much insulin is left. This is because the insulin can degrade and become less effective after this period. In summary, the combination of priming and the limited lifespan of the insulin can cause your pen to run out faster than expected. It's important to follow the instructions for priming and storage to ensure you're getting the correct dose and that your insulin is effective.