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

Poisoned Mithril throwing knives are a type of throwing knife with a poisoning effect. They are made by using weapon poison on a mithril throwing knife.

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.