What happens if someone uses an expired epi-pen?

Inadequate effect: The main problem is that epinephrine, the medicine in the epi-pen, degrades over time and becomes less active. That's a real problem when you are relying on it to save you from an allergic reaction. So if you have nothing else, it's better than nothing, but make it a point to keep your epi-pens current. You can use the old ones to practice using the device by injecting an orange or grapefruit.

May be ineffective: It is unlikely that anything bad will happen except it may not work as well as a new epi-pen. There is a window to check the clarity of the medication and make sure it is clear, or it may be contaminated or the product has precipitated out of solution. I don't recommend that you rely on an expired product in a life threatening reaction.