Cannabis is a natural product, and as with all natural products, it has a finite stability. Cannabinoids decompose, volatile terpenes slowly come off, and plant material can succumb to mold if humidity is too high. It is known that THC decomposes to CBN and that the more volatile monoterpenes usually come off cured plant material faster than heavier sequiterpenes. The following is going to be a literature review that dives into the fundamental question of how long cannabis is good for.
The first paper being reviewed is this paper by Trofin et al. describing their research into the simple question regarding the long term stability of cannabis oil. This research involved a case study of cannabis oil over 4 years in darkness at 4°C and in light at 22°C. In both cases the level of delta 9 THC decreased sharply, the level of CBN steadily increased, and the level of CBD slowly decreased. It may be surprising that there wasn’t that much of a difference between in degradation of delta 9 THC when stored in light at room temperature compared to it being stored in total darkness while refrigerated. In the first year alone, the level of delta 9 THC decreased by around 22% of its original content. After 2 years, the delta 9 THC level was around 44% of its original level. Another interesting result is that there must be another degradation product in addition to CBN due to incongruence between the amount of delta 9 THC lost and the amount of CBN increased. The lack of an equal amount of CBN being observed points to a degradation pathway for CBN and / or an additional degradation pathway for THC even when compensating for the mass difference between THC and CBN.
Research performed by Fairbairn et al. describing the stability of pure cannabinoids over a period of 2 years in varying conditions of temperature and light supports the claim that light is more harmful to THC cannabinoid potency than the difference between room temperature and refrigerated conditions. Unfortunately, the complete study is unable for free access, but the authors proclaim that light was the single greatest factor in the degradation of cannabinoids with the effect of temperature up to 20°C (room temperature) being minimal. The overall claim is that cannabinoid concentrates should be reasonably stable for 1 to 2 years, which is contradicts the results found in the previous source show an approximate a significant decrease of approximately 22% after 1 year and 44% after 2 years.
A more recent study in 1999 performed S.A. Ross and M. A. Elsohly examined the cannabinoids content of cannabis plant matter over 4 years in closed barrels that were stored in a dark, air conditioned (20°C-22°C) storage area. Samples were collected and analyzed annually. The results of this study showed that “on average, the concentration of THC in the plant material decreased by 16.6% ±7.4 of its original value after one year, 26.8% ±7.3 after two years, 34.5% ±7.6 after three years and 41.4% ±6.5 after four years.” This data follows a similar constant trend presented by Trofin et al. A linear decay that is only based on time makes it possible to estimate the age of a cannabis sample. Although the initial THC level is dependent on the strain and plant growth conditions, an estimate of age of cannabis plant material can be achieved with the ratio of CNB to THC. The reports of this paper found that a CBN to THC ratio of 2.5 ±0.9, 6.7 ±1.4, 9.4 ±1.7 and 14.2 ±1.2 are indicative of 1, 2, 3, and 4 years of age respectively.
All of the previous research has focused on the stability of CBD and THC, whereas there have not been much research regarding the carboxylic acid cannabinoids (THCa, CBDa, THCVa, CBDVa, CBNa, and CBCa) or the other minor cannabinoids of CBDV, THCV, CBC, or GBG. I would say that the stability of the carboxylic acid cannabinoids is not of importance because their first decomposition product would be their decarboxylated form of THC, CBD, THCV, etc. However, the stability data of these cannabinoids is of high importance when it comes to accurate quantification in testing. A degraded amount of THCa in a lab’s analytical standards would result in inaccurately high results of THCa as well as inaccurately low results of THC. Manufacturers of these analytical standards give stability data over the course of 4 weeks, at varying temperatures and these carboxylic acid cannabinoids decrease around 2-6% every 4 weeks at room temperature. Due to their fragile nature it is recommended that these analytical lab standards be kept in a -80°C sub-freezer for maximum shelf life or in a -20°C freezer that decreases the expiration date to 12 months.