Ah, interesting questions [@Luminosity] hehe. You're beginning to dig into the plot a bit, but I like the way you're thinking. 

First, HP can only refill a few ways. You can be healed through magic, healed through potions, and healed when the party camps (or gets to a city). Nobody will have access to potions when we first begin as they will be introduced through narrative means. Camping will be a sort of regular thing as the party will be traveling the world eventually. Since this is the medieval era, it'll take some time to reach cities and things so there will be points where the group will camp in which case everyone's HP will be replenished fully. Because there is no combat in cities, entering a city and getting to an inn will also heal the HP of the character going to said. These are mechanics I'll explain just before we begin once this is closed down to new members because there's actually a free-roam mechanic involved. 

This is the more interesting question and kind of digs into the plot a little; does magic poison the person who it is used on in addition to the person who used it? For this moment, I will say no it does not. Magical poisoning occurs when absorbing the energies from the air to mold and create the spell you want to create. Once formed, the spell itself will manifest whatever effect was intended. If it's to heal, it will heal, but it won't poison because the energy has already been formed and molded.