Are There Limits to the Range of Possible Cannabinoid Ratios in Plants?: Page 6 of 6

March 12, 2020
Abstract / Synopsis: 

Strikingly similar results have been reported from a wide range of studies on the ratio of tetrahydrocannabinol (THC) to cannabidiol (CBD) concentrations in strains of cannabis. Whether the source has been legalized markets in the west, medical markets in the U.S. and Canada, or collections from law enforcement and researchers, three easily distinguishable types of plant have consistently been found: THC-dominant strains (with less than 1% CBD); CBD-dominant strains (less than 1% THC); and balanced strains with comparable concentrations of both substances. Another consistent finding of these studies, carried out in a variety of laboratory settings, is a positive correlation between THC and CBD levels in those plants that can make substantial quantities (>1%) of each. The correlation between THC and CBD quantities in these varied populations suggests that there is a fundamental property of the plant that makes some combinations impossible, for instance, >15% THC and also >5% CBD. Such results have never shown up in published data sets of carefully, consistently tested samples, but those were all relatively small collections. A much larger data set has been released by the state of Washington (140,000 flower samples), and this has been scrutinized for evidence of consistently propagated strains with higher than a 2-to-1 ratio.

Looking for consistent representation of certain strains or growers in the “slope” subset produces something of a mixed bag of results. Considerable dispersion exists in both strain names and growers: the final set of 354 entries contains 271 different strain names and come from 179 separate growers. Of the different strain names, only eight represent more than 1% of the total population, and several of these names are just as common in the population as a whole as in this select subset. 

The 354 entries being investigated for reliable evidence of plants that fall between the clusters are just 0.24% of the total number of flower test results in the full database. Of these, only 43 are reported as having >15% THC and >5% CBD. In this most select subset, there are a few entries with repeated strain names, but each of these has its own data issues. In one case, there are four entries with the same strain name from the same grower, but these four samples, tested over 2 weeks in 2015, were entirely unlike the other 110 samples from that grower tested over the subsequent 3 years, all of which had conventional blended (1:1) THC and CBD values. It is possible that strain spuriously produced plants high in both THC and CBD in one instance, but that strain name does not provide a reliable guide to finding it again.

Similar problems are encountered when looking at results from individual growers who had more than one entry with >15% THC and >5% CBD.  One strain name from a grower with two entries in this subset had a strain with a THC value of 16.8% and CBD of 5.5%, but all of the other data about this strain makes this appear to be a simple clerical error: 23 other tests of this strain name, from three growers, were reported as CBD-dominant. This is particularly made clear when we see that on the same day of the results in question, another laboratory tested the same strain from the same grower and reported reverse values: 5.5% THC and 14.4% CBD. Another grower had two entries in this set, but one strain had samples tested by different laboratories two weeks apart, one with 19.1% THC and 1.5% CBD and the other with 24.2% THC and 5.9% CBD. The second test was done by the laboratory flagged by Jikomes and Zoorob for reporting THC results “significantly higher (p < 0.001; Wald test) than all other labs,” suggesting this could be a case of a spurious laboratory result.

A search for reliable strain names or growers in the set of entries with >15% THC and >5% CBD did not prove fruitful, casting doubt on the idea that such strains can be consistently produced, if they exist at all. There are some promising hints, however, that there are reliable plant characteristics in the range of >15% THC and >2% CBD. There are a few strain names that are over-represented in this subset, though they are not exclusive to it: Lavender and Green Crack have enough entries in this sub-set, without obvious data problems, that they are promising leads to continue a search for strains that fall between the clusters.


The lack of reliable evidence for strains with cannabinoid concentrations that fall outside of the three clusters (THC-dominant, CBD-dominant, and balanced) does not necessarily mean that such strains cannot exist; after all, absence of evidence is not the same as evidence of absence. It may be the case that growers have  concluded that there is not a market for strains with 15% THC and 5% CBD, and so have not attempted to initiate or propagate any plants with that combination. However, with all of the incentive for innovation in competitive markets and the interest in new experiences, it seems that someone would try to address this niche. The evidence from the very large Washington data set is that if such a strain is possible, it is a spontaneous occurrence which has not been reliably propagated.

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About the Author

Thomas A. Coogan, PhD, is an Academic and Research Liaison with the New Jersey Cannabis Industry Association. Direct correspondence to: [email protected]


How to Cite this Article

T.A. Coogan, Cannabis Science and Technology 3(2), 32–39 (2020).