Any healing source on the field will instead hurt the Pokemon for the amount healed. Non-hold items are not effected.
Have leftovers? You take damage every turn. Used wish? Half your health is gone when it's granted. Water Absorb? Hah, I'll launch water type attacks at you for bonus damage. Oh, I know Heal Pulse? Hee hee hee...
Of course, it effects that Pokemon and it's allies too, so it's risky.
On 12/02/12 I said I wouldn't change this until something impossible happened. Now, Earthbound just got re-released in the US.