Melanie Leahy has many photos of her son, Matthew. But it is the one of him aged 13, fresh-faced and dressed in his grammar school uniform, that is the most painfully poignant of all.

‘Matthew had just been awarded top marks in all his academic subjects and was on the cusp of adulthood, with his whole future ahead of him,’ she recalls. ‘Even as a boy he spoke about his future, of having a large family and how he wanted to start up his own business.’

Tragically, it is a future Melanie will never see for her only child. For Matthew, 20, previously a Grade A student as well as an expert skier and qualified lifeguard, hanged himself last November. His  suicide was triggered, doctors believe, by smoking cannabis.

Any suicide is an utter tragedy, but what makes Matthew’s case even more shocking is that the young man took his own life while an in-patient at a mental health hospital.

Despite being on hourly observation because of his unstable state of mind, Matthew had managed to hang himself.

‘When I got the phone call to tell me about Matthew’s death, it ripped my heart out,’ says Melanie, Mathew’s mum.

Several other young people have taken their own lives while in the care of the trust in recent years and Matthew’s parents are still waiting for answers as to what really happened to their beloved boy.

It’s cannabis, a drug many class as ‘harmless’, that they blame for his spiral into self-destruction.

Born in December, 1991, Matthew was a ‘healthy, happy and bright lad’ according to his mother.

‘Matthew was absolutely gorgeous and it soon became clear he was very bright,’ says Melanie, who lives in Heybridge, Essex. ‘He learned to swim at the age of three and, by time he was nine, was excelling academically at school.’

‘We were incredibly proud when Matthew passed his 11-plus exam and won a place at Westcliff High School For Boys, a selective grammar school. He excelled at maths and science and was a natural with computers,’ Melanie says. However, by the age of 14, teachers reported that Matthew wasn’t paying as much attention at school.

Because of the characteristic smell of cannabis on his clothes, and his erratic behaviour, both Melanie and Michael suspected he was smoking the drug.

‘I confronted him about it, but he would deny it,’ says Melanie.

By the age of 15, Matthew was having problems sleeping.

‘Matthew saw his GP on his own and I now know from his medical notes that he admitted he was smoking cannabis, although because his friends were also smoking it he told his GP he believed it was harmless,’ says Melanie.

‘This time the doctor advised him about the risks and gave him advice about stopping smoking.’

But Matthew continued and, by the time he was 17, having left school with a clutch of GCSEs and the equivalent of two A-levels, he began his own computer support business. He moved into a shared house that Melanie owned, but at the same time his symptoms worsened significantly.

‘Terrifyingly, he began saying he felt as if something was crawling over his body,’ recalls Melanie. ‘I researched it and pointed out to Matthew it was an effect of cannabis smoking

[doctors call this sensation ‘formication’ and it is one of a number of hallucinations cannabis can trigger].

‘It became so severe that, at the end of 2009, he was referred by his GP to the drug and alcohol early intervention team. They came and spoke to him about the effects of cannabis.’

The drugs team explained he was delusional and Matthew’s state of mind deteriorated so much he was unable to continue his computer job.

‘Eventually in May, Matthew was in so much distress about these imagined parasites that it was preventing him from sleeping properly,’ says Melanie.

Yet he continued to smoke cannabis and his paranoia only worsened.

That June, having experienced a terrifying psychotic episode where he believed worms were destroying him, Matthew was admitted to the psychiatric ward at Clacton hospital and then the Linden Centre, where he remained for five months.

Matthew was given drugs to calm his paranoia, but when he was released in November 2011 the family were shocked at his appearance.

But his problems were clearly far from resolved when, in April 2012, Matthew – by now back in the shared house – erupted at Melanie when she forbade him to grow cannabis there.

Tragically, five days before his death, Matthew rang his father, distraught, saying he’d been drugged and was desperately unhappy.

And finally, he took his own life.