Yeah, kind of! Though eres actually get easier to use the more you, y'know, use them. Once you really master a spell, it sometimes takes less time to cast, less of your energy, and you can even sometimes cast it again right away after executing the first one. But of course, it's still tough on anyone in the long run, but....hmm... I guess the best way to describe it is like taking a really long, hard run. You get tired, but after some rest and maybe a bite of food, your strength comes back. Nothing too bad happens when you're outta juice completely.

You just can't use your eres until your strength's returned. But there's gels and different bottles for that.