My sister have been having this chronic cough?

since she was a kid she had been having heavy cough... the docs said its due to an allergy but they can't determine where it's coming from... they prescribed antihistamine aside from the usual anti cough and phlegm... she's now almost 18 y/o but her cough keeps on recurring which hinders her from studying well... anyone who can help how can this be treated? thanks much! God bless