I have lots of alchemical brews

Can I adminster medicines to companions and NPCs - and

is there a way to administer poisons in food or drink

Healing hands spells work but don't believe there's any way to administer a potion.

in Oblivion I used poison apples from the DB. it was great fun. shouldn't we have them in skyrim too.

If you give potions to NPC's they will use them. This certainly works with health potions but I don't know about buffs. My understanding is that enchantments won't buff skills on NPC's and maybe this true with potions as well. For non followers you would have to drop-pocket into their inventory. Never tried it myself. There is a pickpocket perk that lets you drop-pocket poisons and NPC's will consume them... sort of a Shady Sands Shuffle with poisons instead of grenades.