Alcohol
Don't like the taste, don't like the feeling, don't like the next morning, and I make horrible decisions on it.
Pretty much everytime I drink, I feel like the more I have the better it will be, however that nice buzz will quickly fade (feels like in a turn of the head) and will be replaced by a very bluesy depression. It's not like I'm upset about anything, and prior to it I actually feel like I'm in a very 'fun' room, when suddenly this vacuum sucks out all the fun and replaces it with an emptyness.
I don't drink anymore, mainly due to this reason. Feeling like I've been poisoned is the other part.