From https://www.dol.gov/general/topic/benefits-leave/fmla

The Family and Medical Leave Act (FMLA) is a law passed in January of 1993. The FMLA provides eligible employees with up to 12 weeks of unpaid, job-protected leave per year. The employee’s health benefits must also be maintained during these 12 weeks.

According to the United States Department of Labor, if you would like to take FMLA leave to enroll in a drug rehab program (or for any other purpose), you must meet the following requirements:

You must be employed by a covered employer
You must have worked for your employer for at least 12 months
You must have at least 1,250 hours of service for your employer during the 12 month period immediately preceding the leave
You must work at a location where your employer has at least 50 employees within 75 miles

See More Info at https://www.dol.gov/whd/fmla

Every employer’s FMLA policy is different, so your employer may require that you request FMLA leave within a certain time frame before leaving for rehab. Your employer may also request certification from a health care provider or a second or third medical opinion, at their expense, not yours.

To determine the terms and conditions of for a FMLA leave, contact your Union Rep or Employee Assistance Program EAP.