Macrolides are the best drug therapy for whooping cough.Erythromycin,azythromycin and clarythromycin are commonly used- preferably the latter 2 as erythromycin has some unpleasant side effects.

Erythromycin,azythromycin and clarythromycin are the most used drugs in this case.

Diagnosis and Treatment

http://www.cdc.gov/pertussis/about/diagnosis-treatment.html

Pertussis (Whooping Cough)

http://www.cdc.gov/pertussis/index.html

PREVENTION IS BEST: Vaccine 5 boosters or injections. How the  patients cought with or without treatment around 2 or 3 month. If you want made money with an antitussive herbal tretment, isn´t ethics, only give water the best fluidificant of mucus. Cought is a defensive reflex of respiratory tract. READ: Recommended Antimicrobial Agents for the Treatment and Postexposure Prophylaxis of Pertussis

http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5414a1.htm

Azithromycin,but it is said to reduce infectivity,but does not help the poor child.Actually pertussis is probably grossly underdiagnosed.One study in England found on testing that many children who coughed for a long period actually had whooping cough.