But Government Chief Pharmacist Anthony Chan Wing-kin said although vitamins and hormones had to be registered, melatonin was exempt from the checks on manufacturing, analysis and labelling requirements.

'Melatonin is a unique hormone. It has not been proven to be medicinally effective,' Mr Chan said.

'It only came to Hong Kong two or three years ago and, at that time, we considered whether it should be controlled as a drug like other hormones.' A study of scientific literature had found no dangerous side effects and analysis of some brands of melatonin had raised no concern, Mr Chan said.

But Professor Pang said worrying side effects were coming to light and called for checks on the quality and labelling of melatonin sold here.

'We don't know if the pills you buy over the counter are pure,' Professor Pang said.

'If they [manufacturers] tell you it contains one to three milligrams, how can you be sure? There are no regulations. Also, because it is sold as a health food, there are no directions on how to use it.' Melatonin is banned in Britain and New Zealand but increasingly popular in Hong Kong, Professor Pang said.

'Melatonin has a soporific effect. People become drowsy and want to go to bed,' he said.

It is a synthetic version of a hormone produced by the pineal gland shortly before sleep, and should be taken two to five minutes before going to bed.

'But on some of the bottles it tells you to take it three times a day. If you're driving - what's going to happen?' Professor Pang said.

Doctors were reporting broken bones in elderly people who took melatonin hours before going to bed, then grew drowsy and fell over, he said.