Mephedrone (a.k.a. "bubbles", "drone", "miaow miaow", "meph", "M-CAT" and "white magic") and other cathinone derivatives will become illegal as Class B drugs under the Misuse of Drugs Act 1971 after midnight tonight.
Class B drugs carry a maximum prison sentence of five years for possession and a maximum prison sentence of 14 years for supplying and other trafficking offences.
Drugs already categorised under Class B include amphetamines ("speed") and cannabis.
Until now, mephedrone has been widely available online, often described by sellers as a "plant fertiliser" or "plant food" and marked "not for human consumption". It has become increasingly popular among young people as a cheap recreational drug - it can be acquired for as little as £3 per dose - and has drawn a large amount of press attention over recent months.
The government's former drugs tsar, Professor David Nutt, has criticised the classification of mephedrone as a Class B drug, saying it is a "knee-jerk reaction" over the "supposed problem" which has been "whipped up" into a hysteria.
Professor Nutt, you may remember, was sacked late last year after saying LSD, ecstasy, and cannabis were less harmful than alcohol and tobacco.
A factsheet on the University of Sussex Students' Union website says mephedrone can be snorted or swallowed in capsules or 'bombs' (wraps of paper), or dissolved in liquids. It can also be mixed with tobacco and smoked.
Similar to the already illegal drugs ecstasy and cocaine, mephedrone may produce a feeling of euphoria and increased wellbeing, decrease social inhibitions, and increase energy and talkativeness.
But the drug also has a number of adverse effects, including heart palpitations, mental illness and sometimes death.