'Legal highs' are recreational drugs sold over the internet and the so-called 'head shops' all over the UK. They are freely available to buy and use as they are not covered by the Misuse of Drugs Act 1971. Mephedrone (4-methylmethcathinone) was sold as a 'legal high' until 17 April 2010 when it was made a class B drug under the Misuse of Drugs Act 1971. Numerous deaths and self-harm has been associated with mephedrone use. Effects of mephedrone are reported to be empathogenic similar to 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and stimulant properties similar to cocaine. Not much is known of the effects of mephedrone on mental health. We present a case of dependence and psychosis in a patient using mephedrone (4-methylmethcathinone). The patient needed inpatient hospital care, was treated with antipsychotic olanzapine and recovered well.