I have been coughing for a while and my dad who is a doctor has told me, maybe i should try to get this medicine called promenthazine codiene, he said because it is a liquid and helps faster than most cough medicines. is he correct to a extent?

Codeine is a medication that works on the cough reflex center of the brain to mute its activity. The promethazine is just an antihistamine that thickens mucus. In many respects a cough is a protective mechanism that helps move mucus & debris up from the deep areas of the airways. If your cough is disruptive this may make life easier. If not, why bother?