The propagation of drugs on the road was the main reason for focusing on the development of the rapid methods which are used to detect the presence of drugs which may have been taken. Oral fluid has become a popular specimen to test for the presence of drugs. In vitro tongues were prepared with different concentrations of hydroxypropyl methylcellulose (HPMC E4M); one of these concentrations was chosen to be used for tests. Chemical reagents were prepared which included: cobalt thiocyanate, fast blue B test, Marquis, Mandelin and Zimmerman reagents which were used to examine eight drugs in three different concentrations, each of 10.0 mg/mL, 5.0 mg/mL and 1.0 mg/mL where each reagent was used for a particular drug. The drugs were amphetamine, cannabis, cocaine, 3,4-methylenedioxymethamphetamine (MDMA), codeine, diazepam, heroin, methadone and morphine. Each drug was tested by a specific reagent. The difference in the concentrations gave various results in terms of achieving positive results and the ratio of the clarity of the colour. The amount of the drug on the tongue was between 20 μg and 400 μg. Positive and negative results were obtained in this study. Most of the high concentrations gave positive results; however, the low concentrations gave different results which were between positive, negative and light or very light in colour. In conclusion, there is an explanation of the difference in sensitivity of the effects of the different kinds of reagents in the drug, such as cobalt thiocyanate which was more sensitive at the low concentration of heroin and gave a clear result.