The use of non-prescription drugs is widespread and has a major impact on the health of the individual user and society. In 2006, the British Crime Survey reported that 10% of adults had used one or more illicit drugs in the preceding year, with 3% reporting using a Class A drug. Over 11 million people in the UK are estimated to have used an illicit drug at least once in their lifetime (35%). Drugs abused vary in their intrinsic potential to cause addiction and, with it, more regular and harmful use. Drug users are influenced by trends and fashions, adopting new compounds such as crack cocaine and experimenting with routes of ingestion. Some drugs may become less popular over time, such as LSD, while others, such as cannabis, experience a revival as more potent strains (e.g. Skunk) are developed. A problem drug user is best defined as a person whose drug taking is no longer controlled or undertaken for recreational purposes and where drugs have become a more essential element of the individual’s life. The true economic and social cost of drug use is likely to be substantially greater than the published figures, which are derived from a variety of health and crime surveys which may overlook vulnerable groups such as the homeless. The majority of non-prescription drugs used in the UK are illegal and covered by the Misuse of Drugs Act 1971. The drugs most commonly abused gave rise in 2003–4 to an estimated financial cost in England and Wales of 15.4 billion pounds to the economy, with Class A drugs such as heroin and cocaine accounting for the majority of this. Some 90% of the cost is due to drug-related crime, with only 3% (£488 million) due to health service expenditure, which is mainly spent on inpatient care episodes. This still represents a major health pressure, which in 2006–7 amounted to 38 000 admissions, in England, for primary and secondary drug-related mental or behavioural problems, and over 10 000 admissions recorded for drug poisoning. Clinicians in all specialities can expect to encounter harmful drug use, especially those working in primary care, A & E, and psychiatric services. Presenting problems are protean, ranging from mood disorders, delirium, and psychosis to sepsis, malnutrition, and hepatitis. Blood-borne infections such as hepatitis C and HIV are widespread, as contaminated needles and syringes are shared by up to a quarter of problem drug users. Even smoking drugs such as crack cocaine can lead to increased transmission of hepatitis C through oral ulceration and contact with hot contaminated smoking pipes. Amongst the UK population, over half of IV drug users have hepatitis C, a quarter have antibodies to hepatitis B, and, by 2006, 4662 had been diagnosed with HIV. Non-prescription drug abuse is a leading cause of death and morbidity amongst the young adult population (those aged 16–35). In 2006 there were 1573 deaths where the underlying cause was poisoning, drug abuse, or dependence on substances controlled under the Misuse of Drugs Act. The vast majority (79%) were male. Young men, in particular, are at greater risk of violent death through associated criminal activity such as drug supplying and from deliberate and accidental overdose. The male-to-female ratio for deaths associated with mental and behavioural disorder is 6:1.