'The government has two aims. One is to lower the number of drug abusers via the programme's deterrent effect, the other is to identify abusers in an early stage, but there is evidence showing that neither of them can be achieved,' said Dr Lam Ming, a consultant psychiatrist in Castle Peak Hospital's substance-abuse clinic who co-wrote the paper with fellow psychiatrist Dr Cheung Wai-him.

The paper cited a study in the US state of Michigan of 75,000 schoolchildren from 1998 to 2001 that indicated a testing programme was not associated with the prevalence or frequency of drug use in schools. It said there was little evidence that a voluntary drug-testing programme could identify users early.

'What is more, only 60 per cent of students signed up for the Tai Po scheme,' Lam said. 'The other 40 per cent may be active drug users who might never be tested.'

Timing was also crucial, he said.

'It is well established that the window for detecting most drug abuse is 72 hours or less. For those who signed up, if they take the drug they may refuse to take the test at the last minute and only take the test when they know they are safe.'

'The US launched a similar programme in 1995 and there are lots of studies showing doubts about its effectiveness, so why do we in Hong Kong have to waste money on it now?' Lam asked.

He said the key question was not how to identify abusers but how to help them. 'It is easy for teachers and social workers with a little bit of experience to tell which students are drug abusers. The problem is that we do not have the facilities to treat them when we find them.'

He said there were only about a dozen psychiatrists dealing with substance abuse in public hospitals and that doctors in the city's medical undergraduate programmes received limited training on drug abuse.

'Doctors in general private clinics do not know how to treat these drug abusers at all,' he said.

Explaining why the paper was published only yesterday, he said they finished it in October but missed the deadline for the journal's November issue.

In one of yesterday's drug-related court cases, Vicken Chiu Wai-kan, 23, was sentenced to life in jail for murdering his stepfather under the influence of ketamine, which he had been taking since he was 16.

In the other, model Monique Chau Hoi-ying, 17, was ordered to take a urine test before being sentenced for possessing ketamine in the lavatory of her school, Maria College in King's Road, North Point, on November 17. The Form Five pupil had pleaded guilty earlier and was due for sentencing yesterday, but Acting Principal Magistrate Bina Chainrai delayed sentencing to January 5 for an up-to-date urine test and background reports. Chau was granted bail.

Undersecretary for Security Lai Tung-kwok said the government was aware of the psychiatrists' paper.

'We welcome views from different groups,' he said after attending the launch of a healthy living campaign at the Tai Po Civic Centre.

'As the trial scheme is very new, we need to gain some experience, which will be valuable when extending the scheme to other districts.'

On the lack of professionals in the field, he said it took time to train specialists. 'We need to collect data and do planning for supporting measures to go with the scheme. We will continue to collect views from different professional groups.'

Hospital Authority figures show that at the end of March, 288 doctors, 1,880 psychiatric nurses, 37 clinical psychologists and 131 occupational therapists were serving in public hospital psychiatric units.

The authority also runs a two-year nursing programme in psychiatry for 30 pupils, and the first batch of students will graduate in late 2011.

Lai said it was satisfying to see 12,387, or 61 per cent, of the 20,332 Tai Po pupils who, with their parents, returned forms asking whether or not they consented to drug testing agreed to submit to the scheme.

'We do not set a target,' he said. 'We believe students who join the scheme will spread a positive message to all students in Hong Kong.'