People who buy medicines such as Viagra and Prozac online are playing "Russian roulette" with their health, according to an investigation into the booming internet trade in prescription drugs.

Websites are springing all over the internet offering people the chance to buy anything from slimming to sex pills without the inconvenience or embarrassment of seeing a doctor. But trading standards officers warned yesterday that some of the drugs may be counterfeits made in illicit laboratories that are contaminated or contain varying levels of active ingredients. They also say there is a one-in-four chance the products will not even arrive.

Regulators in Britain, who investigated a selection of drug websites in the UK earlier this month, are battling to control the multimillion-pound business. Within minutes and for a few pounds, computer users can bulk-buy dozens of pills for complaints they may wish to keep private. Popular prescription drugs for sale are the sex treatments Viagra and Levitra, slimming pills, sleeping tablets, baldness cures and anti-depressants.

In their investigation, trading standards officers from Oxfordshire and Worcestershire ordered 20 purchases from foreign websites, ranging from Viagra and Tamiflu to Prozac and Valium. In most cases, the drugs were ordered without any questions being asked. Many arrived with no instructions or information about the supplier or manufacturer. One drug, suspected of being a fake, is being tested. Another five test purchases have still not arrived, more than a month after they were paid for.

The Royal College of General Practitioners said misuse or excessive doses of the drugs obtained in the exercise could cause serious damage - even death. Dr Jim Kennedy, chair of the Royal College of General Practitioners Prescribing Group, said: "Prescription drugs are called that for a reason - they have significant risks and adverse effects if used improperly.

"The list of prescription drugs obtained by trading standards all have a potential for abuse or misuse and could cause side effects and adverse symptoms."