It causes the sap affliction I believe to your favorite bad monster your throwing it at.

Sap isn't very strong except over a very very long fight.
Try casting it on yourself to see how relatively .... not strong it is.

Believe thats magic > submagic > Bleed > targets monster > hit R1 or L1 twice(to target your active party members > pick a member.. > cast

It's very very slow at killing things.

Then just cast esuna I believe, or go to a save crystal to remove it.... or restart the game if you didn't do much

Yes I have cast it a few times myself, and now I won't touch it ever again.