Wolfsbane has a variety of effects on werewolves, apparently depending on variety. Derek used it when burying Laura to keep her from shifting out of wolf form. When Scott was exposed to it in Stiles's car, he developed difficulty breathing and could not control his ability to shift.
Nordic blue monkshood, a rare wolfsbane variety, is able to kill a werewolf in 48 hours.
In Season 4, the Orphans introduced a new variety of wolfsbane that was yellow. When exposed to this variety, werewolves are knocked unconscious and then begin to have seizures. The effects begin relatively quickly and can be fatal within a few hours. In order to prevent the werewolf from dying, one must cut a hole in their chest to release a plume of yellow gas. It is unclear if the pressure build-up is related to their fatality.
Later, Kate Argent is shot with a yellow wolfsbane bullet, but it does not have the same effect. And Peter is kept unconscious using it, although it is not explained why he does not go into seizures and nearly die as Brett and Liam did. Liam's low dose made him weak, but didn't knock him out, however he still needed to have the gas expelled surgically.
The Argents developed a hollowed out bullet that contains Nordic blue monkshood. When shot with this bullet, a werewolf has 48 hours to live, presuming the shot wasn't in itself a killing blow. The wolfsbane acts as a poison, sapping the werewolf of their strength. As it spreads, black veins thread through their skin. Once the poison reaches their heart, they will die.
To counteract the toxin, wolfsbane of the same variety must be burned and the ashes pressed into the wound.