Not the best idea though, as having a needle on the pen provides an opportunity for insulin to leak out of the cartridge and air to leak in, and worse, a pathway for bacteria to get in and contaminate the insulin inside the cartridge, making the insulin not work as well and possibly even lead to a skin infection. Since we already say you "really should" use a new needle every time, then you really really should take the needle off the pen when you're done injecting every time.