You absolutely should follow up with treatment through your group health insurance. Workers' Compensation law permits treatment in this manner and you give up none of your rights under the law. The workers' compensation insurance may be denying your case on the basis that you were simply walking down the stairs. Additional facts are critical to establishing that your accident arises out of your employment.
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In Illinois, it is against the law to threaten, harass or fire an injured worker because he/she filed a workers' compensation claim. State laws differ in the area of workers' compensation and you must consult with an attorney in your state for the best possible advice. In addition to any State law that prevents a termination of employment, you may be covered by a union contract commonly called a collective bargaining agreement.