Cannabis test. Bags of confiscated marijuana about to be analysed as evidence against suspected drug dealers. This is the Duquenois test for cannabis. It uses vanillin in an alcohol solution along with a strong acid (such as the sulphuric acid marked by a label here). Sometimes chloroform is used as an organic solvent to confirm the results of the test. Cannabis is a drug prepared from the plant Cannabis sativa. The active chemical, called THC (tetrahydrocannabinol), is detected by this test, a positive result being a dark purple colour. The effects of THC include euphoria, relaxation and an altered sense of time. Cannabis is illegal in most countries.