dextromethorophan is a cough suppressant; guaifenesin is a mucous-thinning medication that can actually increase a cough. if you goal is to stop/control a cough especially at night then dextromethorophan (assuming it is not included in a combination product containing a decongestant) is what you would use. if you goal is to thin a cough so that a person can spit it up then guaifenesin (during the day) would be helpful if the person is capable of coughing up phlegm. children often cannot do this. guaifenesin at bedtime will most likely increase the cough because of post-nasal drainage.
when do you use dextromethorphan vs guaifenesin