I believe marijuana needs an active blood stream and respiratory system for it to place its effects through inhaling.
Therefore, it would have no effect on the zombie since its bloodstream and lungs do not operate.
Not sure how the brain of a zombie works, that is, if the entire brain serves to promote its ghastly instinct, or if the parts unused as a human are revealed to be favorable for the existence of the undead, whether intentional or not…so, probbaly nothing.
However, if the zombie’s biological disposition was compatible with the effects of marijuana, it would most likely make no difference, as its instinct would overrule the effects of weed.
First of all, the physical effects of weed induce lethargy and hunger-how is that any different than an actual zombie?
Second, the zombie would be immune to its psychological factors, as it has no known awareness other than to eat, and I doubt such a substance would unlock whatever awareness it MAY have.
So, it can’t be freaked out or amazed or dumbfounded, as it’s essentially a walking mass of rotting flesh, and the physical affects of weed can’t make it any worse than it already is.