An oral fluid collection device is provided that includes a borosilicate glass collection tube that can be capped post-collection for containing a human donor's expectorated oral fluid. The collection tube has a lyophilized reagent disposed therein that is essentially free of surfactants and solvents and includes a bacteriostatic, a peptidoglycan cleaving enzyme, an esterase inhibitor, an antioxidant, and a buffer at a pH range of about 5.7 to about 6.5 for stabilizing drugs and drug metabolites that may be present in the donor oral fluid. Interaction between the collected oral fluid and the buffer-preservative is provided by gravity drawing the collected oral fluid downward into contact with the lyophilized buffer-preservative. The lyophilized buffer-preservative brings the collected oral fluid to a pH for stabilization of the drugs and drug metabolites including Δ9-tetrahydrocannabinol (THC), the major metabolite of THC, 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid, (THCA), cocaine, and the major metabolite of cocaine, benzoylecgonine (BZE).