Heroin is the popular name (originally a brand name,- 'Hero-like', in German) for diacetylmorphine or diamorphine, which is synthesized from morphine, a derivative of the opium poppy. It is a strong pain-killer, and also has powerful euphoriant and anxiolytic effects (i.e. it produces feelings of wellbeing and freedom from worry). After many years of abundant supply at high user-end purity in the UK, heroin quality now varies significantly.

Heroin can be a highly addictive drug, producing tolerance to its effects and deep physical dependence—without it the user becomes ill and unable to function normally. It should also be noted that the drug is extremely effective at numbing both psychic and emotional pain, and most addicts can be accurately described as people self-medicating against traumatic memories and intolerable feelings. The time taken to become addicted varies with the individual and context, as does the severity of withdrawal symptoms. Overdose can be fatal, of course, and most overdoses are due to profound depression of the respiratory system ). After many years of abundant supply at user-end purity, in the UK heroin quality now varies significantly.

Street heroin is usually 'cut' or mixed with other powders to bulk it out and increase profits. Purity normally ranges from 30% to 40%. Some of the percentage shortfall may be naturally occuring compounds of the poppy or by-products of the production process, as oppossed to cutting agents. Adulterants may be harmful, but most evidence suggests this is not typical or deliberate supplier policy. It is bad commerce to poison your customers.Periodically, very pure heroin may be sold on the street, potentially causing overdoses due to the heroin being more potent than expected.