Some users believe that the higher the THC level of the strain is, the more powerful the high will be. However, the situation is not that simple. The effects of the strain depend not only on the amount of THC and CBD but also on their ratio.
Apart from THC and CBD, cannabis also has other compounds that influence the results of consuming it, but their role is usually negligible comparing to the psychoactive THC and therapeutic CBD. Both of them interact with brain receptors, and it is THC:CBD ratio that decides how one or another strain will affect your mind and body.
The large number of strains provides us with multiple variations of the THC:CBD ratios as well as countless variations of effects. The most interesting thing is that we can create a new strain by combining the properties of two or more strains. This is where genetics takes the stage, a fascinating science that can help us design the strain with certain properties.
The actual THC and CBD are not present in the plant, they appear only after decarboxylation, which happens when you heat the weed with a lighter, vaporizer, or in the oven. Originally, the plant contains the acidic form of these compounds, THCA and CBDA, respectively. Although the effects of these molecules are very different, they are made from the same precursor, which is called cannabigerol, or CBG.
Before decarboxylation, THCA and CBDA cannot interact with the human cannabinoid system. It is the reason why consuming raw cannabis will have no effect on your state, regardless of how much of it you eat (it will positively affect your digestive system as raw cannabis has lots of fiber, but that is probably not the effect you expect from marijuana).
CBGA can transform into either THCA or CBDA, but not both of them at the same time. The decision of which compound it should synthesize depends only on the two enzymes. When Enzyme 1 interacts with CBGA, it helps it transform into CBDA, while Enzyme 2 converts the same compound into THCA. That is why the THC and CBD level in every strain actually depends on the presence of each of the enzymes: some of them have E1, others—E2, while some strains have both.
With rare exceptions, cannabis plants inherit both genetic lines of its parents. Since the E1 and E2 are contained in two variations of the same gene, the plant has two identical genes that may encode different enzymes. Thus, a plant can have:
This is exactly how the THC:CBD ratio appears.
In the first situation, when only E1 is active, CBGA can turn only into CBDA. It is when you get strains similar to Charlotte's Web and ACDC. They have a high CBD level and no or negligible levels of THC.
The second situation creates the conditions for producing strains rich in THC. E2 from both of the parent strains activates CBGA and transforms it into THCA, which results in strains like Girl Scout Cookies, Jack Herer, and generally most of the strains that you can find in dispensaries. Due to the high level of THC, these strains are popular among recreational users.
The third situation can produce a great number of versions since the exact amount of E1 and E2 is variable. The new strain can produce THCA and CBDA in similar amounts, like it happened, for instance, with Harlequin and Afghani CBD strains, or can give preference to one or the other enzyme, creating strains like Cannatonic or Blue Blood.
However, you need to remember that THC and CBD are not the only compounds that determine the strain's profile. These three possibilities only explain the matter of the THC:CBD ratio.
For modern cannabis strain genetics, only THC and CBD levels are essential as they are studied the most. While CBD-rich strains are commonly used for medicinal and industrial purposes, THC-rich cannabis is popular for recreational consumption. Regulating the balance between these two compounds helps growers get the desirable effects and avoid the negative side effects.