Cough relief should last about 12 hours. Guaifenesin, the expectorant in Mucinex DM, is rapidly cleared from the body, having a half-life of only one hour. A drug's half-life is the amount of time it takes for the body to eliminate half of the drug.
Urine: DXM can be detected in urine for up to 2 days after the last use, but standardized drug tests may detect it as PCP (Angel Dust). Blood: DXM can be detected in blood anywhere from 3 to 24 hours after the last use.