WASHINGTON (Reuters) - Employees can use dozens of different products and techniques to beat drug tests, but their employers can detect most of them if they know what to look for, an expert said on Monday.

Household chemicals, herbal teas and even drinking gallons of water can dilute or neutralize traces of banned drugs in urine, said Amitava Dasgupta of the University of Texas-Houston Medical School.

"It is estimated that approximately 20 million employees are screened each year in the United States for illicit drugs. Marijuana is the most frequently abused drug in the United States," Dasgupta said in a written summary presented to a meeting of the American Association for Clinical Chemistry in Washington.

A mini-industry has arisen to help people hide drug use that could cost them their jobs, Dasgupta said.

"Common household chemicals such as laundry bleach, table salt, toilet bowl cleaner, hand soap and vinegar have been for many years used as adulterants of urine specimens in an attempt to avoid a positive drug test," he said.

Using chemicals like potassium iodide can reveal such tactics, he told the meeting.

"There is also a popular belief that drinking goldenseal tea helps to escape detection of an abused drug," he added. Such tea may help dilute the sample, but he said it darkens the urine to a characteristic color.

"More recently a variety of products have become commercially available which can be ordered through Internet sites," Dasgupta said.

"Synthetic urine is available from these Internet sites as a sure method to beat a drug test in settings where collection of a urine specimen is not supervised."

Salt, liquid hand soap and drain cleaner can mask barbiturates, while certain eye drops can mask benzodiazepines, a frequently abused anxiety drug.

If testers know of these tactics, they can easily circumvent them, Dasgupta said. "Detection of adulterated urine specimens using spot color test is relatively inexpensive and can be easily adopted in clinical laboratories," he said.