Blood and plasma cannabinoid stability is important for test interpretation and is best studied in authentic rather than fortified samples. Previous studies employed green-top (sodium heparin) tubes; however, gray-top (sodium fluoride-potassium oxalate) tubes are generally used in forensic cases. Three participants each provided three low and high blood and plasma pools after smoking a 6.8 % Δ9-tetrahydrocannabinol (THC) cigarette ad libitum over 10 min. THC, 11-hydroxy-THC (11-OH-THC), 11-nor-9-carboxy-THC (THCCOOH), and THCCOOH-glucuronide stabilities in samples collected via gray-top tubes were determined after 1 week at room temperature (RT), 1, 4, 12, and 26 ± 2 weeks at 4 °C and 1, 4, 12, 26 ± 2, and 52 ± 4 weeks at −20 °C. Concentration changes 47 %. In blood at 4 °C, THC, 11-OH-THC, THCCOOH-glucuronide, and THCCOOH were stable for 26, 26, 1, and 26 weeks, respectively, and for 26, 52, 12, and 52 weeks respectively, at −20 °C. In plasma, THC and 11-OH-THC were stable for 1 week at room temperature, while THCCOOH-glucuronide decreased and THCCOOH increased. In plasma at 4 °C, THC, 11-OH-THC, THCCOOH-glucuronide, and THCCOOH were stable for 12, 26, 1, and 4 weeks, respectively, and at −20 °C for 52, 52, 12, and 52 weeks, respectively. Blood and plasma specimens collected via gray-top tubes should be stored at −20 °C for ≤12 weeks to assure accurate quantitative results of all major and minor cannabinoid markers.