Although CBD and THC have the same chemical formula (C21 H30 O2), the two phytocannabinoids differ structurally. CBD has a hydroxyl group where THC has a closed ring. This one structural difference between the two phytocannabinoids leads to significantly different phytocannabinoid receptor binding capacities. (Casajuana Köguel, Crippa, Ligresti, Mechoulam, Zuardi)

Reproduced with permission under the Creative Commons Attribution License from Calapai et al.
Preclinical and Clinical Evidence Supporting Use of Cannabidiol in Psychiatry Evidence-Based Complementary and Alternative Medicine Volume 2019, Aug 2019Article ID 2509129, 11 pages https://doi.org/10.1155/2019/2509129

CBD influences binding at cannabinoid-1 (CB1) and cannabinoid-2 (CB2) receptors in multiple ways, but unlike THC, CBD’s mechanisms of action do not involve the direct binding to CB1 or CB2 receptors; CBD has a very low affinity for the cannabinoid receptors. (Devane, Ligresti, Bisogno)

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