Dragonsbane is a deluding fluid used by Susa to weaken the Serpent at Gaia Rock in Golden Sun: The Lost Age. At first, it doesn't seem to help much against the vile creature because even after feeding vast quantities of the liquid to the Serpent, he still cannot kill it with the Cloud Brand. However, after Felix's battle with the beast proves successful (but only after further weakening it with beams of sunlight), they claim to Uzume in Izumo that the Dragonsbane did in fact cause the Serpent to be less powerful during their fight.
In the Japanese version of The Lost Age, the fluid is simply named Sake, a Japanese alcoholic beverage. This gives it consistency with the legend of Susanoo, who feeds the dragon with loads of Sake to put it in a drunken stupor so that he may slay it. The alcoholic reference was removed for censorship reasons and thus was given the name Dragonsbane for the English release.