is it illegal for an employer to lay off a full time employee while the employer still has temporary employees employed?

It is not clear how many weeks you were absent for your treatment, or whether you had exhausted your 12 weeks of FMLA leave per year. It also is not clear from the information you provided whether your employer is coverd by the FMLA. If your employer employs at least 50 people for 20 weeks in the past 12 months, and if you worked at least 1250 hours in the past 12 months, your employer would be covered and you would be eligible for the benefit. For eligible employees of covered employers, the law entitles you to take at least 12 weeks FMLA leave to deal with a serious health condition. Addiction can be considered a serious health condition. If the employer has laid you off while retaining temporary employees in this circumstance, you may have a suit for unlawful interference with your FMLA leave rights or retaliation for utilizing your FMLA leave rights. Moreover, you also may have a suit for unlawful discrimination based on your disability. Drug addiction (but not current drug use) is protected as a disability under the ADA and Rehabilitaion Acts. You have a history of drug addiction which is considered a disability as well. Also the employer may consider you to be a drug addict. Any or all 3 of those factors is sufficient to make the employer's action in laying you off a potentially successful lawsuit. For ADA/Rehabilitation Act cases you must file a charge with the EEOC first.  You must file it not later than 180 days after the date of the discriminatory act (in your case the date you were informed of the layoff).  To bring an FMLA cae you do not have to file a charge first.  You can initiate a federal lawsuit withoug going through any administrative steps. If you need additional information or assistance you may call me. Michael A. Caldwell 404-979-3154