The UK population was estimated to be 65.1 million according to the 2015 mid-year estimate (Office for National Statistics, 2016d). Eighty-four per cent (54.8 million) live in England, eight per cent (5.4 million) in Scotland, five per cent (3.1 million) in Wales and three per cent (1.9 million) in Northern Ireland. General population surveys (GPS) indicate that cannabis is the most commonly used illicit drug in the UK, and it has been in each year that relevant surveys have been conducted. Treatment demand for cannabis has risen for as far back as there is reliable data, despite prevalence having fallen. Cocaine (in powder form) is the next most commonly used drug, followed by ecstasy.