One of Scotland's leading drug experts has warned that methadone is a "highly dangerous" drug that carries a "very high risk of overdose".

Professor Neil McKeganey, director of the centre for drugs misuse research at Glasgow University, said: "Drinking a quantity of methadone in the absence of a heroin habit is highly dangerous and carries the real risk of fatal overdose.

"Methadone is a powerful depressant that affects the heart and remains in the body for a number of days and would still be a worry."

And a distraught mother spoke of the grief she feels over the death of her teenage daughter, who died from a mouthful of methadone - a year after winning her fight against a heroin addiction.

Linzi McGowan, 19, was found dead at a friend's flat on Sunday morning, less than 24 hours after being released from hospital following a methadone overdose.

Her grieving mother, Myrna, 50, said: "Words cannot describe our grief. It's unbelievable. What gives me some comfort is the thought that my wee Linzi doesn't know she's dead.

"Linzi started on heroin when she was just young. I think it had a grip on her for about a year. She came to me and said, 'Mum I need your help'. She was my daughter, you would do anything for your children."

Linzi refused to take methadone at the time and went "cold turkey" for three weeks at her mother's home in the Edinburgh suburb of Chesser.

Mrs McGowan said she was at a loss to understand why her daughter - who had been working as an admin assistant with Snowsport Scotland - apparently fell back into old habits when her life looked so positive.