krokodil, or "crocodile". It is desomorphine, a synthetic opiate many times more powerful than heroin that is created from a complex chain of mixing and chemical reactions, which the addicts perform from memory several times a day. While heroin costs from £20 to £60 per dose, desomorphine can be "cooked" from codeine-based headache pills that cost £2 per pack, and other household ingredients available cheaply from the markets.
It is a drug for the poor, and its effects are horrific. It was given its reptilian name because its poisonous ingredients quickly turn the skin scaly.