In our group we're allowing a prepared herb to get picked out of a herb pouch and ingest it as a 50% action during a round. In a similar way I think we'd allow a potion to get fetched out of some container in easy reach and drunk. This 50% activity requirement means that melee combat, e.g. parrying, would not be possible. If there was some way for the character to have the potion very quickly available, like some kind of potions belt (never seen such as this), then you could reduce the required percentage below 50%. If you don't like that idea then simply don't allow such equipment.
Just my 2 cents