Two days after the AFL outlined a stringent drug-testing regime for Ben Cousins in his football comeback, Melbourne-based Dr Peter Larkins said some readily available products were successful masking agents for drugs.

Internet websites such as www.smokeface.com offer several products, including hair follicle cleansers and random drug-testing solutions, and offer a money-back guarantee to help users "test negative every single time".

A worker from WA's horseracing industry claimed she and her colleagues had used a "hydroponic flush" to beat tests for cannabis use.

She said pills and a liquid substance, which could be bought over the counter for about $200, had proved successful, as long as they were taken within four hours before a drug test.

While Dr Larkins warned that not all products were legitimate, he conceded some could be used to hide drug use. He said government doctors using hair-follicle testing to monitor drug use in the workplace had confirmed they were aware of such products.

Dr Larkins likened chemical tampering with hair follicles to the ability to bleach hair.

He conceded it was not possible for drug-testing agencies to be aware of all potential masking products on the market.

"There are a range of products on the market that claim to have anecdotal success with being able to interfere with hair-follicle testing, and the people involved in that testing say that it's probably true," he said.

"But if someone wants to go to the trouble of making sure they have the shampoo done, it only clears the hair for a day or two, so you have to know when you're going to have your sample taken.

"If some athlete or person wanted to be so particular about going out and getting the shampoo and doing it to avoid the test, they could probably cheat the system.

"You can detoxify your hair with the right sort of chemicals and we know bleaching penetrates into the hair shaft and follicle, so I've got no doubt that there are some products that can totally stuff up the composition of your hair or even kill it.

"You're clearly playing Russian roulette buying a product to think you can escape detection, because some work and some don't. But I don't think we're ever going to be able to say we've got a test that's infallible."

AFL chief executive Andrew Demetriou yesterday refused to answer questions regarding the products. But he said in August that the governing body's testing regime was not perfect.

The Australian Sports Anti-Doping Authority confirmed yesterday it was aware the masking products were available.

An ASADA spokesman said it routinely analysed samples for masking agents, which were easily detectable through a process known as gas chromatography mass spectroscopy (GCMS).

Dr Larkins said urine tests could detect drug use within the previous 24 to 36 hours, while hair-follicle testing could uncover illicit substances used in the previous three months.

He said hair samples could come from anywhere on the body, except the pubic region because it contained a different oil composition and different glands affecting the blood supply.

While the AFL has committed to a trial of hair sampling, unlike Cousins, those who record positive drug tests will not be punished.