what is the best way to heal companions on hardcoe. Cass got injured by a Cazadore, and her health is down 2/3. Does the health regenerate? Do I have to spend a fortune on stimpacks? Frustratingly, using a doctor didnt give the option to heal her, which would have been nice and cheap.

Probably stimpacks.

What he said. And I may be mistaken (because I only play on Core), but I believe that you can put an Antivenom in their inventory and they'll use it.

Stimpacks will not heal a poisoned companion. Antivenom is the only way to get their health to max back out.

EDIT : I stand corrected, they will NOT use Antivenom, when placed in their Inventory. Check The Vault, under Companions ( bugs), for an explanation of ways to help / prevent a poisoned companion.

Ah, yes. Protip: Play Honest Hearts for 10 billion vials of anti-venom.

Ah, yes. Protip 2: You don't need HH for Antivenom. Your first bottle is on a shelf at Doc Mitch. Then It's a campfire and a 40 Surv Skill.

It's an easy question. Pick fight with bloatfly or any other weak critter. Allow Cass to kill said critter to make sure she's goes into combat mode. Cass will now be back to 100% health.

I must admit, props goes out to ZombieUnicorn...It DOES seem that the Datura antivenom can be used by Companions, to heal poisons.

Good Job!

Just keep in mind that companions can't pause the game like the courier, so they first finish the fight and only then use antivenom.
If you are playing on hardcoe make sure the fight ends fast, otherwise they will die regardless.

I wish there was an option in the wheel for applying antivenom on them though.