New research into the selling of drugs online has found that, while the number of sites offering drugs has declined for the first time since 2004, overall availability is still too high.

Researchers at Columbia University's National Center on Addiction and Substance Abuse (Casa) found 365 sites this year selling drugs to consumers, compared to 581 over the same period last year.

"This decline in the number of sites advertising or selling controlled prescription drugs may reflect efforts of federal and state agencies and financial institutions to crack down on internet drug trafficking," said Joseph A Califano Jr, president of Casa, and former US Secretary of Health, Education, and Welfare.

"Nevertheless, in spite of those efforts, anyone of any age can obtain dangerous and addictive prescription drugs with the click of a mouse.

"This problem is not going away. It is morphing into different outlets for controlled prescription drug trafficking like internet script mills and membership sites that sell lists of online pharmacies, and different payment methods like eChecks, cash-on-delivery and money orders."

The study found that 90 per cent of sites sold benzodiazepines like Xanax and Valium, 57 per cent sold opioids like Vicodin and OxyContin, and 27 per cent sold stimulants like Ritalin and Adderall.

Overall 85 per cent of sites did not demand a prescription for the drugs, and half of those that did only required the prescription to be faxed, which makes forgery easier.

Most of the drug sites were located outside the US, although almost a quarter are registered in the country. Worryingly, the geographical location of over a third of the sites proved impossible to determine.

The group called for mandatory prescriptions and regulation and certification of internet drug sites, and said that search engines should block all unregistered sites.