Tetrahydrocannabinolic acid (THCA) is just not your usual THC. It's the cannabinoid precursor to THC. When it can be exposed to light or warmth, it undergoes a procedure called decarboxylation, which ends up in it dropping its carboxyl group and turning it into THC. Without THCA, there wouldn't be THC. https://josephl890zxv0.wikibestproducts.com/user