Tang Mei-kwan, 45, unemployed, was convicted in Kowloon Court of possessing a dangerous drug and neglecting or ill-treating a child.

Principal Magistrate Tong Man remanded the weeping mother in custody and called for a drug addiction treatment centre report.

The court yesterday heard that Tang used to be an outpatient at the Yau Ma Tei Methadone Clinic. When she visited the clinic in the evening of December 12 last year, the staff gave her about 40mg of methadone.

Methadone is a medication used to help recovering drug addicts overcome withdrawal symptoms.

Tang put the methadone into her mouth but then spat about 10mg into a plastic bottle, which she put in the pocket of her pants and took home.

Tang wore the same pants when she went to bed with her son.

At 11pm she saw her son licking the plastic bottle and saw traces of methadone on the floor and around the boy's mouth.

Tang cleaned the floor and put the bottle into the fridge.

She observed her son for a while but he seemed to be normal. The child fell asleep around midnight after drinking some milk. But four hours later, Tang discovered that her son was unconscious and his lips had turned black.

The boy was taken to the intensive care unit at Queen Elizabeth Hospital but he was in coma. He was diagnosed as having opioid intoxication and methadone was found in his blood and urine.

Before the incident he had been developing normally.

Marcel Koo Wing-leung, associate professor of the department of pharmacology at the University of Hong Kong, said the child had suffered from an overdose of methadone, which could cause brain damage.

Tang told police it had been her 'careless mistake'.

Defence counsel David Chan said Tang, who had five convictions, with one related to drugs, was very remorseful for her negligence.

He said the single mother, who had a disabled leg, a Primary Four education and had not worked for seven years, was living on welfare.

'She knows she is responsible for her offence of possession of methadone. But the biggest lesson is that her 22-month-old son was hurt in the incident. She is very remorseful,' Mr Chan said.

'The son is under the care of the grandmother and she wants to be united with her son quickly so she can take care of him.'

The case was adjourned to September 8 for sentencing.

In April this year, a couple of recovering drug addicts were jailed for 31/2 years for manslaughter, neglect of a child and possession of a dangerous drug after their 16-month-old baby girl died after drinking methadone they had stored in the fridge.