When you use the following command:
/effect @e minecraft:instant_damage 9999 255 true
the effect happens to all entities except for the player.
MC-84462
Potion of Harming doesn't work on self when typed in on the commands console
Resolved
Duplicate
0
D O
(Unassigned)
Unconfirmed
Minecraft 15w31c
Retrieved
If you're in Creative, you won't take any damage from it.