I tried potions, spells such... Nothing works! I'm now lv33 and I still can't do anything about poisoning, this is getting ridiculous (and I'm currently exploring the devil's cavern so my character's portrait is often green).

there is a cure spell, it's not working ?

Nope...

PURGE
DISPELL MAGIC
these speels work sometimes

A companion automatically could drink potions to heal itself.

But they don't seem to do the same with cure potions.

Is there any way of cure a companion of venom effect? (Main character is a thief).

PURGE. When i'm poisoned & fear, it always works.

Thank you very much pal.

You must have a high Magic of True Faith skill, because Purge is far from 'always works'.