A TEENAGER stabbed his grandmother to death as she lay naked in the bath after he became paranoid and depressed, a court has heard.

Pietro Addis stabbed Sue Addis in the bath 17 times at her home in Brighton on January 7 last year.

Dr Peter Misch, a psychiatrist instructed by the defence, has told the court that he interviewed Pietro Addis several times following the stabbing, during which the teenager told the doctor he believed that his grandmother was “trying to kill him”.

This psychotic episode, said Dr Misch, would “undoubtedly be the cause of Pietro killing his grandmother”.

Three of the interviews took place while Addis was at HMP Cookham Wood and Bluebird House, a secure mental health facility for young people, following the teenager’s arrest last year.

Pietro Addis has admitted killing his grandmother but denies murder.

“What came across consistently during the interviews was a sense of paranoia,” said Dr Misch.

In the weeks prior to the killing, the teenager was suffering “severe anxiety”, he added.

“During [that time] he couldn’t even make a phone call to order a pizza. He had been too scared to open the door.”

In a statement read to the court, Addis’s friend George Cameron said he noticed a change in the teenager in the months leading up to the stabbing of the popular 69-year-old restaurant owner.

He said Pietro had become "very negative" and despondent with life and believed he "had no friends".

"Pietro has always been popular. He had a lot of people who cared about him but he was feeling isolated," Mr Cameron wrote.

He said his friend had been having problems with his dad, Leo.

"There was a lot of conflict at the house ... Pietro was using cannabis and Xanax [an anti-depressant] to ease his pain."

The jury has also been told that a hair sample taken from Addis, who was 17 at the time he stabbed his grandmother, also showed traces of cocaine, MDMA and ketamine in his system.

The court has also heard that he had taken his grandmother’s breast cancer medication before the killing.

George Cameron added: "I was shocked when I find out what had happened to his grandmother. The Pietro I knew was not the Pietro who did this.”

Dr Misch said the evidence of George Cameron was ”significant”.

“This is a boy who describes his mate changing quite profoundly,” he told the court..

The trial at Lewes Crown Court continues.

READ MORE: Recap - Teen had 'psychotic belief grandmother was trying to harm him' - trial told