Potions and poisons with both positive and negative effects

Using alchemy I combined Blue Mountain Flower and Hanging Moss to create a poison that had Damage Magicka Regen (100%) and Fortify Health effects, i.e. a positive and a negative effect.

Does this mean if I use it to poison an enemy then they will not be able to regen magicka but will gain health?

Also, is it possible to craft a potion with both positive and negative effects? If so, how is it decided if a poison or potion is created?

Solution 1:

Yes, you can create potions that do harm to you in addition to their positive effects and conversely create poisons that help the enemy as well doing their negative effect.

The level 100 Alchemy perk, Purity, counters this making it so that potions do no negative effects and that poisons do no positive effects.