RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Poisoned Mithril Spear
The Old School RuneScape Wiki also has an article on Poisoned Mithril Spear.

A Poisoned Mithril Spear is a poisonous spear which can be made by using weapon poison on a Mithril Spear. It can be thrown or used in melee combat.

They have no level requirements to wield, which may be a glitch, as the non-poisoned version requires level 20 Attack and level 15 Ranged to wield.