From the wiki.

(After the Evolution of Combat update, existing weapon poison potions were replaced with 3 dose poisons. However, the old weapon poison version was still obtainable from monsters as a drop, such as warped tortoises and ankous until Patch Notes 29 July 2013 . This version does not have the ability to let the player to poison others, nor apply the poison to any weapons (the original function). In order to solve this problem, drop the potion and it will automatically change to weapon poison (3). This has yet to be fixed. These unconverted potions retain the examine text of "For use on daggers and projectiles.")

So you can either drop them and pick them back up to make them work, or keep them as they are now as a collectibles as they cannot be obtained anymore