Smack Rat

Smack rat. A person who has an appearance strikingly similar to that of a rat and just so happened to partake in copious amounts of smack (opioids)

Often a cold hearted fellow who would give their left arm for a packet of baccy and a spam fritter.

(Say in a vicious and raspy voice as an insult)

Person-Here, I wish they would stop being so horrible
Person 2- Don't worry mate, they're a fucking Smack Rat