In the dried plant, THC is mostly present as THCA, an acid form that is not intoxicating. Only when it is heated, for example when smoked or vaporised, does part of the molecule split off, and THCA turns into THC.
Labs therefore measure both and add them together:
Total THC = THC + 0.877 × THCA
An example: if a sample contains 20 per cent THCA and 0.5 per cent THC, that makes 0.5 + 17.5 = 18.0 per cent total THC.
The factor 0.877 comes from chemistry: THCA loses weight when it converts, and of 100 parts of THCA, around 87.7 parts of THC remain. Total THC therefore shows the maximum amount of THC that can be produced if all the THCA is converted. The same calculation applies to CBD and its acid form CBDA.
This total value is what counts. A low THC value on its own says little if there is a lot of THCA.
What the value does not tell you
The total THC value tells you the maximum amount of THC that can form from the flower. It only partly tells you how strongly it will affect you. It says nothing about purity: whether a sample is free of mould, germs, pesticides or heavy metals is only shown by separate tests.
The amount counts. The more THC reaches your body, the stronger the high is on average. How much reaches it depends on how much you take and how you inhale. When smoking, between 2 and 56 per cent of the THC entered the blood in studies. People who smoke stronger cannabis often inhale less, but usually do not fully make up for it. In studies, people who often use cannabis that is very high in THC are more often affected by dependence and psychosis.
The form of use counts. In one study, the same amount of THC had a stronger effect when vaporised than when smoked. When eaten, it often only takes effect after 30 to 90 minutes, but then lasts for many hours.
You count too. People who use regularly feel the same amount less strongly. In studies, women felt more on average from the same amount, also independently of body weight. Alcohol raises the THC level in the blood and increases impairment. Expectations, mood and surroundings also play a part.
CBD does not make THC less harmful. In one study, up to three times as much CBD made no difference to the memory impairment and psychosis-like symptoms caused by THC. When eaten in large amounts, CBD can even intensify the effect of THC.
Terpenes are substances that contribute to the smell of the plant, such as myrcene or limonene. In flowers, they make up around two per cent on average. Whether they change the effect of THC has not been established. The so-called entourage effect is a hypothesis. So far, there are two small studies on this in humans: one found no effect, the other found one only at an amount above the usual content of flowers.
Strain names and the labels “indica” and “sativa” say little about what is in a flower. In one study, people who use cannabis could not tell from the smell how much THC a sample contained.
Remember: The total THC value is what counts for the risk. How strongly it affects you depends on the amount, the form of use and you.
More on this: What cannabis does in the body · Lab resultsDE
Sources
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