A post nasal drip will make you cough. You can get this from a cold or from allergies. There are many different types of Robitussin so I don't know what you tried. To stop the cough, you need to stop the drip. You many wish to try an antihistamine for this. If you don't get results, I would recommend that you see your doctor. Perhaps the eye infection cleared up so well with visine and boric acid because it was an allergy. Your doctor will be able to find out what is causing the post nasal drip and give you a stronger medicine. If this is helpful, please check accept. Best of luck.