I have a persistent cough that been going on for too long?

What is "too long?": A cough productive of sputum requires medical attention. A non-productive cough that has not cleared within a week might have many causes and also requires evaluation by a trusted physician. I like Robitussin (over the counter) or Elixir Turpin Hydrate with Codeine (by Rx).

Evaluation: You should see your doctor to be evaluated to find the cause so be treated.