Ive used this command:

/execute @e[type=!Item] ~ ~ ~ detect ~ ~-1 ~ soul_sand 0 /effect @e[r=0] poison 2 2

When I stood on soul sand, I do get poisoned, but do not gain any damage. I only get damaged after i walk off the soul sand

Alternate steps to reproduce:

1. Place a repeating (needs redstone) command block with a lever ontop
2. Insert

   /effect @p minecraft:poison 2 0

   in the command block

3. Go to survival/adventure mode
4. Turn the lever on
5. You have the poison harts, but take no damage
6. Turn the lever off
7. You now take damage