Customs officers have seen a sharp rise in the seizure of a tranquilliser taken by mainland schoolchildren who believe it makes them smarter. There has been a dramatic rise in checkpoint finds of the drug Estazolam since a 1992 order for doctors in Hong Kong to tighten dispensing procedures. Estazolam is a member of the benzodiazepine group of drugs commonly prescribed for insomnia. They act as anti-depressants and slow the central nervous system. 'When I arrived about 10 years ago, there was a significant abuse problem of benzodiazepines,' said Professor Julian Critchley, head of clinical pharmacology at Chinese University. 'We had the impression that benzodiazepines were being used as an alcohol substitute by some young people who were intolerant of alcohol,' he wrote in a 1995 issue of Clinical Toxicology. Abuse apparently had been controlled and fresh cases fell from 24 per cent in 1990 to 4.3 per cent in 1992, Professor Critchley said. But the apparent fall in known abuse contrasts sharply with soaring Customs' seizures as tablets are smuggled in from Shenzhen. Last year, 9,570 tablets were seized at Lowu checkpoint. The culprits were all men aged between 21 and 40. Customs officers made no seizures between 1990 and 1992 - just before the tranquilliser was declared a dangerous drug. William Ng Wai-ming, superintendent of the Customs drug investigation group, believes the tablets smuggled into Hong Kong in recent years are not destined for blackmarket trade but for traffickers' friends and families. The pills are sold in Hong Kong for $10 each; in Shenzhen they can be bought over the counter for about one yuan a tablet.