DRUG user Matthew Buswell could be fine one day and lost and confused the next, according to his defence barrister.
Buswell, 23, of Wyke Road, Trowbridge, was jailed for five years after admitting wounding with intent.
Swindon Crown Court heard when Buswell slashed Alex Palmer across the face with a metal tool during a drunken Christmas Eve attack he had seemed uptight and had been acting strangely.
Liz Cunningham, prosecuting, told the court on Thursday how Mr Palmer and Buswell had fought early in the evening and Buswell had gone home, only to return to Trowbridge town centre later to confront his victim.
At 9.20pm a witness queuing outside Chicago Rock Caf, in Park Road, saw Mr Palmer and a friend walk past, followed by Buswell.
Seconds later Palmer ran past, having been slashed around the face.
When questioned by police Buswell initially denied the attack but, on being confronted with CCTV images, accepted he was responsible.
Paul Grumbar, defending, said that Buswell suffers from a personality disorder brought on by the use of cannabis and had been annoyed by events earlier in the evening. Jailing him, Judge John McNaught said: "I am sentencing you for a very serious attack.
"The fact is that night you armed yourself with that instrument and used it to cut someone's face."
As well as a five-year jail sentence Buswell will remain on licence for a further three years.
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