What is psychosis?

A student who killed an academic who had gone out to post cards announcing the birth of his daughter has been given an indefinite hospital order.

Femi Nandap, 23, stabbed Jeroen Ensink, 41, to death in Islington, north London, while in a cannabis-induced psychotic rage last December.

Dr Trevor Turner, a consultant psychiatrist, explained to the BBC's Eddie Mair what psychosis is and whether it can put those suffering from it, or those around them, at risk.