While Having A Cold, My Husband Developed A Horrible Cough

While having a cold, my husband developed a horrible cough that even codeine cough syrup can not touch. After research our doctor advised to stop list optic. The attacks have lessen (not every 1/2 hr) to every 6 hrs or so, but the severity has become frightening where he almost blacks out. And yes, he is taking Claritin for allergies. It has been 5 days since stopping the medication.
How long will this last?