cannabinoids can be degraded by environmental factors like temperature, light or acid but also by several enzymes in our body.
Plant cannabinoids in their natural state are typically acidic. For instance a plant that is reported to contain 20% THC actually contains up to 18% THC-acid or THCA and only ± 2% THC. When cannabis is heated to 100-130 °C most THCA will be converted to THC. This is important to remember as THC and THCA have different therapeutic properties.
When cannabis is heated further, for instance with smoking, up to 30% of cannabinoids can be lost due to combustion.
Everything in our body is eventually degraded by enzymes. cannabinoids are no exception.