Medical illness question

Dear lawguru.com,

I have been diagnosed with a spinal illness that causes severe chronic pain. I have had back surgery and was out of work from December 2005 through March 2006. In March 2006 I returned to work in limited capacity at four hours per day five days a week. Since then I have increased hours worked to currently 7.5 hours a day five days a week. I am limited as to how long I can sit due to increased pain which elevates after six hours even though I am taking strong doses of pain medication. I am currently under a ''Work as Tolerated'' instructions from my doctor. Can my employer take take disciplinary actions against me because I am not working 40 hours per week? I have provided my employer with a doctor's statement of my illness and ongoing treatments.

More than six hours per day of work causes me pain that is above what I believe to be tolerable but I have worked more hours than this for fear of losing my job.

Thank you for your time.