I am taking prescribed narcotics for a broken hand. Will that look bad to the employer even though its legal?

As a manager of a Fortune 500 company, I would certainly not look at this negatively unless it effects your ability to do your job safely. For instance, if you drive as part of your job, I might not want you on the street in case you get sleepy. Everyone has little mishaps and things that happen and most all of us take medication to help alleviate the pain. As long as you can safely get in to work and work without being drowsy, I'd say your employer should have no concerns. Good luck mending that hand.