The stability of cannabinoids is complex and crucial for the assessment of impaired driving caused by cannabis. Therefore, the effect of antioxidants on the long-term stability of Δ9 -tetrahydrocannabinol (THC), cannabinol (CBN), cannabidiol (CBD), 11-hydroxy-Δ9 -tetrahydrocannabinol (THC-OH), and 11-nor-9-carboxy-Δ9 -tetrahydrocannabinol (THC-COOH) in whole blood samples preserved with fluoride citrate (FC) and fluoride oxalate (FX) mixtures was investigated at different temperatures. The measured concentrations of the cannabinoids in authentic whole blood preserved solely with FC or FX mixtures decreased significantly during prolonged storage at -20°C. On average, less than 5% of the initial concentrations of THC and CBD were recovered after 19 weeks of storage interrupted by 5 thawing/freezing cycles. The rate of decrease was greatest in FC-preserved blood. The repeated thawing/freezing of the samples accelerated the instability progression. At 5°C approximately 60% of the initial concentrations of THC and CBD were recovered after 19 weeks of storage. No significant decrease was observed in samples stored at -80°C during the test period of 5 months. The instability at -20°C was to a great extend avoided by adding 30 mM ascorbic acid (ASC) to the samples before storage. Samples preserved with a combination of the FX mixture and ASC showed no significant decrease in the recovered concentrations during a 5-month storage period interrupted by 6 thawing/freezing cycles. Samples preserved with a combination of the FC mixture and ASC showed almost similar improvements in cannabinoid stability. Other reducing agents such as sodium metabisulfite and glutathione also improved the stability in FX-preserved blood stored at -20°C.