If your Company is covered by California Labor Code § 1025, then you will be required to reasonably accommodate the request, regardless of whether your Company policies permit it. The California statute provides, that:
“Every private employer regularly employing 25 or more employees shall reasonably accommodate any employee who wishes to voluntarily enter and participate in an alcohol or drug rehabilitation program, provided that this reasonable accommodation does not impose an undue hardship on the employer.
Nothing in this chapter shall be construed to prohibit an employer from refusing to hire, or discharging an employee who, because of the employee's current use of alcohol or drugs, is unable to perform his or her duties, or cannot perform the duties in a manner which would not endanger his or her health or safety or the health or safety of others."
If your Company employs more than five employees, then you are also covered by FEHA. Alcoholism/drug addiction is likely a disability, and therefore, you will also be required to reasonably accommodate under FEHA.