Oral fluid is composed of saliva, mixed with buccal and mucosal transudates, cellular debris, bacteria, and residue of ingested products. Oral fluid as a test matrix shows promise for detection of recent drug use, and a significant body of scientific literature documents drug disposition and detection times. Researchers comparing effectiveness of oral fluid and urine drug testing found a similar pattern and frequency of positive drug test results in the general workforce over the same general period. Similarly in pain clinics, the pattern of licit and illicit drugs and metabolites observed in oral fluid paralleled results reported for urine, with some minor differences in detection rates for different drug classes. In some cases, oral fluid testing may be an acceptable alternative or the only available matrix. As an ultra-filtrate of blood, however, it shares the limitations of a decreased window of detection versus conventional urine drug testing.