A 15-year-old boy may have battered a man to death because his personality was changed by an antidepressant drug, a specialist doctor has told a court.
The boy killed Gary Belben, 59, with a hammer and attempted to kill his wife, Tanya, in Essex after being prescribed Prozac, Chelmsford Crown Court heard.
The boy, who cannot be identified and who denies murder and attempted murder, was given the drug for depression.
Lawyers representing him argue the drug caused a "mental abnormality".
His lawyers contend the boy is not guilty of murder but guilty of manslaughter on the grounds of diminished responsibility.
People given the drug (can) exhibit turbulent emotions, compelling thoughts
Consultant clinical pharmacologist Andrew Herxheimer said Prozac was one of a group of anti-depressant drugs capable of causing mental disturbance.
He said research showed a tiny percentage of patients could develop obsessions and become emotionally "flat".
"People given the drug (can) exhibit turbulent emotions, compelling thoughts," Dr Herxheimer told jurors.
"There is a compulsiveness about it. An obsessional idea that will not let the person go as the person is in the grip of that idea.
"It is a steady vice-like grip of the person's thoughts and behaviour.
"Emotions seem to be side-lined. They are blanked out by this obsessional thought. One of the effects is emotional flatness."
Martyn Levett, prosecuting, told jurors the boy killed Mr Belben and attacked Mrs Belben in Colchester in December 2007.
Mr Levett said prosecutors would dispute the boy's defence and contend the teenager was guilty of murder and attempted murder.
He said they would argue that the reasons for the boy's actions were more complicated than defence lawyers suggested.
Jurors would have to decide whether the teenager's mental state "substantially impaired (his) responsibility for the killing," Mr Levett told the court.
The trial continues.