Beginning Monday, California employers will be barred from discriminating against employees who use marijuana in their off hours.

Assembly Bill 2188 requires employers to change how they test for marijuana use among employees, using tests that show current impairment and not just past usage. The law carves out some exceptions, such as for those who work in the construction trades.

“Technology to test for marijuana impairment has actually advanced quite a bit, so basically employers can now just test for THC the psychoactive component in cannabis and that can show impairment,” said Los Angeles-based attorney Bernard Alexander in a statement. “Most of the older tests detect the non-psychoactive metabolites, which can stay in a person’s system for weeks. So, a worker can test positive when they’re not high or impaired at all.”

California Gov. Gavin Newsom signed AB 2188 into law in September 2022, but a provision of the law prevented it from going into effect until Jan. 1, 2024.

The bill was championed by California NORML, a marijuana advocacy organization, which said in a statement that “not a single, scientifically controlled FDA study has shown cannabis metabolite testing to be effective in improving workplace safety or productivity. Studies indicate that metabolite tests for past use of marijuana are useless in protecting job safety.”

The group pointed to a survey of more than 136,000 Canadian workers that found no association between cannabis use and workplace-related injury, even in high-risk occupations.

The bill was opposed by the California Chamber of Commerce, which argued that employers would face liability by taking legitimate disciplinary measures against employees who use marijuana.

“Put simply: marijuana use is not the same as protecting workers against discrimination based on race or national origin and should not be in (the Fair Employment and Housing Act),” the chamber wrote in its argument.

Alexander noted that employers who disapprove of employees’ use of marijuana even in off hours “will be looking for alternative ways to terminate employees for other reasons, and attorneys who defend those employees will argue those other reasons are just pretexts.”

Los Angeles-based attorney Amy Duerk said that while the law will protect the rights of most workers to use cannabis when not working, it doesn’t preclude employers from keeping their workplace drug-free.

“These laws will push improvements in testing for impairment and psychoactive metabolites in cannabis. Employers will likely step up efforts to train supervisors on impairment testing. Testing will continue to improve to help employers distinguish between recent use of cannabis and non-impairment traces of cannabis,” Duerk said in a statement.