Oral fluid is an alternative biological matrix that might have advantages over urine for drug analysis in treatment programs. A liquid chromatography-tandem mass spectrometry (LC-MS-MS) method has been used for screening 32 of the most commonly abused drugs and their metabolites in 0.5 mL preserved oral fluid, and the results were compared to results obtained from urine samples taken at the same time. In all, 164 pairs of oral fluid and urine were obtained from 45 patients stabilized on either methadone or buprenorphine. The total number of detections of drugs other than buprenorphine or methadone was 535 in oral fluid and 629 in urine. Morphine was found more often in urine (n = 66) than in oral fluid (n = 48), whereas the opposite was the case for 6-monoacetylmorphine (n = 20 in urine and n = 48 in oral fluid). Methadone showed the same detection frequency in urine and oral fluid (n = 75), whereas amphetamine (n = 45 in urine and n = 51 in oral fluid), methamphetamine (n = 39 in urine and n = 45 in oral fluid), and N-desmethyldiazepam (n = 37 in urine and n = 51 in oral fluid) were detected slightly more often in oral fluid. The other benzodiazepines, cannabis and cocaine were found more frequently in urine samples. If using a sensitive LC-MS-MS technique, oral fluid might be a good alternative to urine for detection of relatively recent use of drugs.