It depends on whether you are still working. If you are eligible for COBRA because you lost your job, just being eligible for COBRA doesn’t affect your eligibility for premium tax credits or cost-sharing assistance if you enroll in a marketplace plan. However, if you are eligible for COBRA because your hours at work were reduced and you are no longer eligible for your job-based plan, you will have to show that your COBRA is unaffordable or fails to meet minimum value in order to qualify for premium tax credits. (26 C.F.R. § 1.36B-2(b) and (c); 78 Fed. Reg. 25909, May 3, 2013; IRS, Questions and Answers on the Premium Tax Credit).