So I saved after a huge battle with Cazcadors, my companion has ~40% of HP left but he's poisoned .. using stimpacks won't help, using antivenom or giving him any won't help. Travelling to town doesn't help.
Delete the game data to get rid of the patch. From what I understand, that's a new bug where poison never wears off of companions, and the version 1.02 patch causes it. Just DON'T delete your save data and you won't actually lose anything (other than about 10 minutes of your time).
I've only encountered this problem after installing the 1.02 patch (US). After going unconscious and waking up, they are no longer poisoned. Sending them to the Lucky 38 also has the same effect. Obviously, it is a huge pain in the arse, especially when you depend on them to kick some cazador butt.
Interestingly with Lily, her HP will only drop so low from poison and for some reason, she apparently is no longer poisoned.
it's probably because when you get done fighting with them, their state goes back to normal, so if they still are on fire or poisoned they remain that way until it would normally wear off. i guess the new patch somehow made it so it's a permanent effect, but when you go into combat again, and they don't get poisoned that time, the poison already wore off before that combat ended, so it isn't there anymore. assuming of course you know what i am saying, but it is really hard to put what i am thinking into words.