robus
Lowcountry Low Roller
DM explains
if they make the save .. the arrow barely misses the player and the poison was never delivered, but the player is a bit more tired because they dodged out of the way just in time. The next one might be their last!
if they fail the save ... the arrow nicked the player .. small scratch but "oh no it's a poison arrow!"
But success on a poison save means half the poison damage is inflicted surely?
From the Giant Poisonous Snake stat block in the MM (pg 327):
Bite. Melee Weapon Attack: ... target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.
I believe it's the same with all poison weapon attacks.