Troll mutagen is an alchemy ingredient and is used to craft troll decoction. They act like lesser green mutagens when placed in the diamond-shaped slots in the character panel.

Like all special mutagens they can only be dropped by their corresponding monster. Almost any troll will do, though the drop rate is quite low and only Wham-a-Wham is guaranteed to drop it.

Notes

• In an undocumented change for Patch 1.08, all mutagen icons were changed. However, there was still a bug for those on Steam and Origin that didn't reflect this and was later fixed in Patch 1.22. Despite this, some may still see the original icons instead, even with a fully patched game.
• A common strategy is to find a lone rabid rock troll, like the one near Trail to Yngvar's Fang signpost, then save before killing it and reload until it drops.
• The ice troll encountered at Clan Tordarroch Forge erroneously drops other loot, while Bart (if killed during Blindingly Obvious) drops no loot at all - they are the only known exceptions who can be killed, but can't drop the mutagen.