Currently, Regen and Poison are opposing each other at a 1:1 rate. This is probably a mistake and will probably be fixed.
Example: Playing two upped Purify gives you a '+4' regen counter. The opponent plays 'Deadly Poison', bringing it to a '+1' regen counter. Then Poison is played, making it into a Poison Counter of '1'. Another upped Purify brings it to '+1', not removing the poison first, and merely adding the '+2' to the '-1'.