How does a doctor differentiates which medicine to prescribe patient between promethazine and guaifenesin?

Good question: Guaifenesin is prescribed to loosen phlegm (mucus) so that it can be expelled more easily. But it's not particularly effective. Phenergan (promethazine) has mild sedative effects which may suppress the cough reflex but it's not specifically approved for that indication. I don't know how or why any specific doctor differentiates between the two. It's a mystery!

Don't use either: Promethazine is an antihistamine used for nausea and vomiting. Also used as a cough suppressant. However it has many side effects and is ineffective for acute coughs due to colds, bronchitis etc...and shouldn't be used.(Cough suppressants in general are ineffective for acute coughs). Guaifenesin is safe and is supposed to help loosen mucus with a chest cold. But in a recent study it was ineffective