A Critical Review of the Interactions Between Δ9-THC and Other Naturally Occurring Phytochemical Constituents of Cannabis sativa.

INTRODUCTION: This narrative review systematically examines the available empirical evidence for interactions between delta-9-tetrahydrocannabinol (Δ9-THC or THC) and C. sativa's other phytochemical constituents hypothesized to underpin the popular yet controversial notion of 'entourage effects' experienced with botanical cannabis. Many relevant preclinical and clinical studies have been conducted in the last decade to elucidate the individual contributions of various phytocannabinoids and bioactive terpenes to the pharmacodynamic effects of C. sativa. However, their varied methods and results complicate their interpretation. Here, a particular emphasis is placed on synthesizing data from clinical studies of THC's modulation by Cannabis sativa's other phytochemical constituents. METHODS: A systematic search of the scientific literature indexed in PubMed was conducted to identify controlled clinical and preclinical studies evaluating the modulation of THC's effects by individual phytochemicals. Studies that evaluated the phytocannabinoids cannabidiol (CBD), cannabinol (CBN), tetrahydrocannabivarin (THCV), cannabigerol (CBG), cannabichromene (CBC), and cannabis terpenes α-pinene, d-limonene, β-myrcene, linalool, α-humulene, β-caryophyllene, bisabolol, terpinene, and terpineol were included in the literature search for discussion. RESULTS: Cannabidiol and THC interactions have been extensively studied, with recent findings showing significant exacerbation of THC effects when orally coadministered, but not when inhaled. Clinical studies examining THC interactions with constituents beyond CBD remain comparatively limited. Recent clinical evidence suggests that THCV may reduce certain subjective and cognitive effects of THC. At the same time, some terpenes, such as limonene, can selectively modulate adverse effects associated with high THC doses in humans. Clinical studies have often failed to translate the interactive effects between THC and minor phytocannabinoids/terpenes observed in animal models. DISCUSSION: Although interest in the clinical pharmacology of minor cannabinoids and terpenes has grown in recent years, the evidence supporting meaningful modulation of THC's effects in humans remains exceptionally limited. The mixed results from preliminary studies underscore the complexity of phytochemical interactions and the inherent challenges of translating preclinical findings into measurable clinical outcomes. CONCLUSION: Well-designed, controlled clinical studies of individual phytochemical interactions on subjective, therapeutic, and adverse effects are still required to support entourage effects in humans. In particular, the effects of many bioactive terpenes and trace cannabinoids have not yet begun to be evaluated in humans, either as individual phytochemicals or when combined with THC.