Does not stack with other elemental weapon effects, such as weapon runes, secondary elemental damage built into a weapon (e.g. Red Grace, Blade of Red Birth), or even the Elemental Weapons effect cast by a different companion.
If cast on a weapon with built-in secondary elemental damage, this spell overwrites its display in the floating damage values. If cast on a weapon with an elemental weapon rune installed, the visual effect applies on the weapon itself but only the damage from the weapon rune is displayed. It is unclear if this affects the actual damage dealt.
As the strength of weapon runes are dependent on the weapon's base damage as well, weapon runes will always provide a larger damage bonus on the same weapon.
If cast prior to applying Deathroot Toxin, the spell's visual effect will overwrite the poison's visual effect and vice-a-versa. It is unclear if this overwrites the other's damage itself; while both effects display as active on the HUD, only one elemental color appears in the floating damage values.