Fired for not disclosing prescription after on the job injury drug test?

First, you have a worker's compensation case and should speak with an attorney for that matter asap.  Second, to determine if you have a claim for the termination will require more facts.  Unless you are employed under certain terms (contract), Indiana is an employee-at-will state.  You can be terminated for any reason as long as it was not solely discriminatory (i.e. age, race, gender, disability).