^ That's called gameplay.
That's called shit design. The point you're gonna find out, how much medicine you need to treat a critically injured follower is the point where you end up with one. This is oftentimes a scripted or random occurence, like the time Isabella gets kidnapped. As deuxhero said, if you find a lot of herbs, you may be inclined to sell most of them.
While you should be able to handle having one guy require treatment, two or three will make you run out of medicine pretty quickly, unless you hoarded a TON of herbs.