Any Poison cannot decrease any stat.
This is because, unlike other status conditions, there are two types of poison: Poison and Bad Poison (Toxic Poison).
Regular Poison takes 12.5% of your HP away every turn, and can't increase or decrease.
Toxic Poison's damage starts at 6.25% of your HP and increases by 6.25% of your HP every turn, and resets when you switch out. This is highly useful, and substitutes for the stats that Poison doesn't lower (such as Speed for Paralysis).
The moves that can cause Bad Poisoning are Toxic (100% chance), 2 Layers of Toxic Spikes (100%), Fling (100% when Toxic orb is held) and Poison Fang (50%).