Daniel de Costa, who is charged with murdering Devizes dad Matthew Baggott by stabbing him 14 times in St John's Court, Devizes, on September 6 2007, had exhibited hyperactive behaviour from the age of two, Winchester Crown Court heard on Monday.

Defence expert witness, psychiatrist Dr Paul Cantrell, told the jury of six men and six women that he had been called in to examine De Costa following his arrest for the stabbing.

He spoke to him in Winchester Prison in December 2007 and also interviewed his mother, who works for the Probation Service.

De Costa's mother, who was not named in court, had told Dr Cantrell that she had campaigned for many years to get her son the help he needed. She had first noticed signs of overactivity at the age of two but it was not until he was 18 that a firm diagnosis of Attention Deficit Hyeractivity Disorder (ADHD) was made.

De Costa's primary school career was marred by frequent anti-social behaviour resulting in suspension and eventual expulsion.

When he was 12 he started abusing alcohol and was drinking one and a half litres of vodka and several cans of strong beer a day by the time he was 16. He began taking illegal drugs when he was 13 and his behaviour continued to deteriorate. He became verbally abusive to his mother and tried to attack his stepfather with a cricket bat.

Finally, when he was 18, he was diagnosed with ADHD and prescribed Ritalin, which his mother described as a "miracle drug".

He managed to pass several GCSEs but it was a brief reprieve and he was soon abusing drugs, mainly amphetamines, but he progressed to heroin and crack cocaine.

Dr Cantrell agreed with defence barrister, Nigel Pascoe QC, that De Costa sold heroin to fund his amphetamine habit.

Then, in March 2007, when De Costa was living in Cornwall, he asked his consultant to let him come off all prescription drugs so he could "detox" himself. Despite serious concerns among De Costa's treatment team he was given substantial prescriptions so he could medicate himself.

He then left Cornwall to return to Wiltshire and in September 2007 the stabbing of Mr Baggott took place.

De Costa denies murder but has already pleaded guilty to manslaughter due to an abnormality of mind.