My Doctor Said I Have A Dry Cough Due To

My doctor said I have a dry cough due to allergies and after several cough medicines and cough drops and even prescribed allergy medicine nothing has worked. He recommended Mucinex DM. I read about it helping you cough up mucus but I'm not congested just this terrible cough. He said my lungs are clear. Will this help me and for how long should I continue to take it? When should I worry that this could be something else?