Dr David Ostleton, of the Forensic Science Service, said the presence of the analgesic codeine in many common medicines and painkillers was leading to people being wrongly accused of using opiates.
He said: "My big concern is that if employers do not do confirmatory testing then there is an opportunity for a miscarriage of justice to occur."
Dr Ostleton, who was speaking yesterday at a Standing Conference on Drug Abuse debate in London on drug testing, highlighted a range of flaws in testing methods increasingly being used by employers.
The use of poorly trained people to carry out testing could easily lead to the contamination of samples.
Testing involving hair samples - rather than blood, saliva or urine - is more likely to show positive against dark-haired people than those with grey hair. Some cannabis smokers use commercially available diuretics products that can sometimes lead to the drug being so diluted it is undetectable.
Drug testing is expanding in a rapid but haphazard manner in workplaces. The UK Standing Committee to Develop Guidelines for Workplace Drug Tests will publish a report in July designed to establish a standard blueprint.
Mike Goodman, of the drug advice charity Release, said more people were contacting his organisation over fears about drug testing than any other issue, including the effects of Ecstasy.
He said: `The number one question by a million miles is `How long does cannabis stay in the system?'"
Other clients had said they had tested positive for opiates after eating poppy seeds on French bread.
Earlier, the UK Anti-Drugs Co-ordinator, Keith Hellawell, said drug testing in the workplace should be an "integral part" of a company's health and safety policy, and should also include alcohol.Reuse content