Arguably the best way to find Madness Mutagens is the infinitely spawning harpies in front of the Visionary's hut. The harpies there deal less damage and only have 50 vitality but their loot drops are the same as the regular version, with enough patience players can pick up 5-6 Madness Mutagens even in Dark and Insane mode.
It is also possible to farm some in Chapter I. The easiest way is by farming endrega, an excellent location for it is near the eastern nest. The endrega there all climb down from the same tree, you can position Geralt right at the base of the tree and wait for them to climb down. Once they do they will have their backs turned on him so you can kill them easily with backstabs. At the beginning of the chapter they will respawn in pairs each time you save/load. Simply save the game then load it, next kill the pair that climbs down and see if they drop the mutagen, if not then reload the same save and try again, if it does drop then save the game and now reload the new save for the next pair to spawn. Eventually only 1 endrega will spawn but only after 8+ waves which is more than enough in that chapter.
Unlike greater mutagens which can be generated via Side Effect, this mutagen appears to be exclusive to monster drops.
- The mutagen has the combined effects of Strength mutagen, Vitality mutagen and Concentration mutagen
- While there have been reports of the mutagen dropping from enemies that drop lesser and greater mutagens, the chance of it happening appears very slim. Since the mutagen is technically a basic mutagen the best chance to find them are enemies that normally drop basic mutagens, like endrega, wraiths, and harpies.