Change your preferences here.
Deals damage and has a 40% chance to poison the target.
Smog is simply too weak to see any competitive use. While the 40% chance to poison the opponent is nice, and higher than any similar moves, the lack of Base Power means that you are better off either using Toxic or a different Poison-type move. Interestingly, a multitude of Fire-types get this attack; however, the one main type they would want to hit with this move would be the Grass-type, and their STAB already scores a super effective hit on said type.
Sludge Bomb is a more powerful alternative, with only a 30% chance to poison the target.
Sludge Wave is a more powerful alternative, with only a 10% chance of poisoning the opponent.