Instant magic effects are not always instant
One sure way to see this is by creating an on-touch spell which contains both "Poison" effect and after that "Cure Poison" effect, and casting the spell on an NPC. Get their health everytime before casting the spell on them to notice how the cure poison effect doesn't work instantly, it should never allow the NPC's health to be damaged by poison here.
Edited by Abdu Sharif