Cannabis is a widely used recreational drug obtained from the plant Cannabis sativa. The plant produces chemicals of which the most psychoactive agent is Δ9-tetrahydrocannabinol (THC). THC is metabolized in the body to THC-OH and then to the psychotropically inactive metabolite THC-COOH. The current method using GC-MS requires extensive samples preparation and long run times. We have developed a method to measure THC and its metabolites on the LC-MS/MS for forensic use.