I'm using doxycyline exp 2014 with good effect!
#notmedicaladvice
Excluding food, the vast majority of use-by dates mean they might become less effective, not useless or dangerous. So use more if concerned! It's the alcohol-induced evaporation which causes bug-killing, so long as it's still doing that, then go nuts.
Note to self - do trial with in date and out of date alcohol gel then swab hands after to count bacterial colonies .... one day when I get bored at work .....
Edit for pushka: Yes some medications are quite unstable (thyroxine quite known for this - hence often needing refrigeration depending on formulation) and this definitely doesn't apply!