Troll potion is a quest item used in the Eadgar's Ruse quest. It is made by drying out a Troll thistle on a fire to make a dried thistle, crushing it in a pestle and mortar and then adding it to an unfinished ranarr potion. You get 77.5 Herblore experience from making it.
The potion is a truth serum that the player must use on a Troll called Burntmeat in order to find out information about Goutweed.
After the quest, the potion can no longer be created. Using ground thistle on an unfinished ranarr potion produces in the message "You don't need to make any more."