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Termination of an employee in retaliation
for that employee exercising their right to workers compensation is illegal. However, if an employer can prove other legitimate reasons for the termination, then termination is not retaliatory in nature. Filing for workers compensation does not somehow make an employee "immune" from termination, and if that employee tested positive for drugs in violation of company policy, that would typically constitute valid grounds for termination, regardless of whether a workers compensation claim had been filed.
Termination of employees for prescription marijuana use is a contentious issue in the state of California. Specifically, new legislation has been in the works to outlaw discrimination in the workplace on the basis of medical marijuana usage. The bill is Senate Bill 129, which you can read the text of here: http://www.leginfo.ca.gov/pub/11-12/bill/sen/sb_0101-0150/sb_129_bill_20110127_introduced.html
To my knowledge, SB 129 has not yet become law and may never. Accordingly, it would appear that the present state of the law is that an employer is typically still free to terminate someone on the basis of marijuana use, even if that use is lawful and for a legitimate medical condition.
So to summarize, while it is illegal to terminate an employee "in retaliation" for filing a workers compensation claim, a termination is not "retaliatory" and thus not illegal if there is another legitimate basis to let the employee go, such as drug use. Prescription marijuana drug use presently remains a valid basis for termination.
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