I know there are already two answers, but I have a couple more things to add. Poison does damage at the end of every turn. It doesn't lower any stats. Also, if a Pokemon is badly poisoned, the poison does more and more damage at the end of each turn. Basically, no. Poison does not lower any stats. However, it does power up the power of Hex (if your opponent is poisoned) and Facade (if you're poisoned). However, note that the other status conditions do this as well.
Hope this helped.