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The Limits on THC and CBD Levels

For decades, growers have used selective breeding to create strains with ever-higher levels of THC. Some have done the same to produce low-THC, high-CBD profiles. But is there a limit on the THC or CBD levels that cannabis can produce? Or will growers continue to pack more power per pound into the plant?

The answer is simple: There are limits. And today’s growers have probably already reached the natural limits with the most potent THC-dominant and CBD-dominant strains. Because CBD and THC are ultimately derived from CBGA, and strain genetics limit how that happens, there are serious biological constraints on the THC:CBD ratios that strains can have.

Like most plants and animals, cannabis plants inherit two copies of their genes. As it turns out, the E1 and E2 enzymes that turn CBGA into either CBDA or THCA are encoded by two different versions of the same gene (in reality things are a bit more complicated than this, but the end result is the same). Because each plant gets two copies of that gene, there are only three possibilities: A plant can have two copies of the gene that encodes the E1 enzyme, it can have one copy each of the genes that encode E1 and E2, or it can have two copies of the gene that encodes E2.
Genetics + cannabinoid enzymes graphic

The genetics of a cannabis strain determine whether it produces only CBDA, only THCA, or both. The E1 enzyme converts CBGA into CBDA, while the E2 enzyme converts CBGA into THCA. Two versions of the same gene encode E1 and E2. A plant can have two copies of one version of this gene, or it can have one copy of each. (Photo credit: Leafly)

Importantly, these three possibilities are based solely on the THC:CBD ratio, and don’t take into account other compounds that a particular strain might produce. The three broad THC:CBD ratio strain categories are:

CBD-dominant strains (hemp): If a strain gets two copies of the gene that makes E1, only CBGA to CBDA is possible. These strains will have high levels of CBD and negligible levels of THC. Charlotte’s Web is the most famous strain in this category.
Balanced strains: If a strain gets one copy each of the E1 and E2 genes, it will produce CBDA and THCA at somewhat similar levels. Let’s call these “balanced strains”—they produce both CBDA and THCA. Strains like Harlequin or Cannatonic are good examples.

THC-dominant strains: If a strain inherits two copies of the E2 gene, only CBGA to THCA is possible. These strains make up most of what we see on dispensary shelves today. Blue Dream, Girl Scout Cookies, and the bulk of commercial strains fall into this category. They have high THC levels and negligible CBD.

Cannabis genetics limit THC and CBD production so that only these three broad categories of flower are possible. Hemp strains do not produce significant levels of THC, while most commercial strains fall into the THC-dominant category—they have THC but negligible levels of CBD. “Balanced strains” produce both THC and CBD, but generally not as much THC as THC-dominant strains or as much CBD as the more potent hemp strains.

But there is an upper limit. The biological limits on THC production mean that ~35% total THC by dry weight is a rough upper limit for strains. On average, high-THC strains contain ~18-20% total THC, while the more potent strains will contain ~25-30% total THC. You should almost never see a strain with more than 35% total THC by dry weight. Be skeptical if you do.
(Note: in the next article of this series, we’ll look more closely at what Total THC by % dry weight means, and how to read it on labels.) Mixed strains are also limited in how much CBD and THC they can produce. They can’t produce as much THC as the more potent THC-dominant strains, and they can’t produce as much CBD as the more potent hemp strains. 20% total CBD by dry weight would be considered highly potent for a hemp strain.

Most balanced strains will tend to have CBD and THC levels in the neighborhood of ~6-12%. But you should never see a strain with 30% THC and 10% CBD, or one with 30% of both. The biology of THC and CBD production prevents this. Be skeptical if you see flower claiming to contain anything very far from the shaded purple regions in Figure 1.

Why Do These Limits Exist?

If you think about it, there are commonsense reasons why these limits exist. When appropriately labeled, cannabis products tell you how much THC, CBD, or other compounds are present as a percentage of the dry weight of that product. If the flower you buy is labelled as having 25% THC by dry weight, it means that 25% of the mass of the flower, after it has been dried to remove water, comes from THC. That means 75% of its dry weight comes from other things: fats, carbs, proteins, vitamins, and minerals. All that stuff. More THC or CBD means less of those things, which the plant depends on for survival.

But just knowing the % dry weight number isn’t enough if you want to know exactly how much THC you will really be dealing with when consuming your favorite strain. That’s because a lot of the “THC” in your product is really THCA, much of which will be converted into THC by the heat from your lighter, vape or oven. And this process isn’t 100% efficient, which means things aren’t so cut and dry that you can just read the numbers on the label.

No plant, however, will produce large amounts of either THC or CBD if it is not genetically predisposed to do so. Giving your plant optimal environmental conditions is the best way to coax THC, CBD, and other cannabinoids out from a strain. Yet, doing so would only help the plant produce to its fullest potential, it would not inspire it to produce something that was not already made possible by genetics.

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