Its fine...been there done that myself. As long as it is still clear it is fine. Remember Lantus is meant to be used in humans and part of the reason it is in the pen form is so they can just pop it in a purse or pocket and take it with them. The only reason we keep it in the fridge is to extend its life since kitties use it slower than humans. Now it may start to peter out faster than a pen that never got left out, but testing will tell you when it starts to lose its punch.
Mel, Maxwell, Autumn & The Fur Gang