And research by the National Drug and Alcohol Research Centre has found that the "silk road", an online black-market trading site, is expanding at a similar rate. A senior lecturer at the University of Tasmania, Raimondo Bruno, said the size of the online drugs market, and the risks posed by the drugs, scared him. "People are dabbling in a wide range of substances we don't really have a lot of human data on," he said. "Some of these products are known to be neurotoxic, even in animals." Dr Bruno said there appeared to be two types of sales: specific chemicals, for which more than 78,000 searches originate in Australia each month, and "blended" drugs, for which about 18,500 searches are made. He said the blended drugs, sold under generic brand names, were particularly unpredictable. "Almost all the drugs we're talking about haven't had any history of human testing - a couple are failed antidepressants or other drugs," he said. "

Because the blends don't state the content, or they state misleading content, you can buy something one week and go back to the same seller and buy the same brand and it can be different." Dr Bruno will present his research at the Australasian Professional Society on Alcohol and other Drugs Conference in Melbourne on Wednesday. The executive director of the Australian National Council on Drugs, Gino Vumbaca, said the new drugs posed a significant risk, particularly as long-term health outcomes were unknown. "A lot of these drugs are marketed as a legal alternative, and people think they're legal and that there's less risk," he said. "They assume it's gone through some kind of pharmaceutical procedure." He said research indicated most of the new compounds were coming from China and India, and retailers knew if someone was hurt by their products there were plenty more users to take their place.

A researcher at the National Drug and Alcohol Research Centre at the University of NSW, Joe Van Buskirk, who is also presenting at the conference, said he had found an average of about 15 new sellers a fortnight on the silk road willing to ship drugs to Australia. The silk road exists on what is known as the ''dark'' or ''hidden'' web, operating on a membership basis and using an alternative currency. "In order to sell anything, or even buy anything, you have to gain the trust of the forums beforehand,'' Mr Van Buskirk said. International and domestic sellers sold there, although international sellers offered cheaper prices. Loading He said the drugs sold tended to be older drugs such as cannabis, which he was surprised to find was the most common product.

Figures from the Australian Customs and Boarder Protection Service show cannabis is the second most common drug caught in cargo and postal importation, with the number of captures increasing by 86 per cent in the past three years.