The use of the intoxicating cannabinoid delta-8-tetrahydrocannabinol (Δ 8 -THC) has grown rapidly over the last several years. There have been dozens of Δ 8 -THC studies dating back over many decades, yet no review articles have comprehensively covered these findings. In this review, we summarize the pharmacological studies of Δ 8 -THC, including receptor binding, cell signalling, in vivo cannabimimetic activity, clinical activity and pharmacokinetics. We give special focus to studies that directly compared Δ 8 -THC to its more commonly studied isomer, Δ 9 -THC. Overall, the pharmacokinetics and pharmacodynamics of Δ 8 -THC and Δ 9 -THC are very similar. Δ 8 -THC is a partial agonist of the cannabinoid CB 1 receptor and has cannabimimetic activity in both animals and humans. The reduced potency of Δ 8 -THC in clinical studies compared with Δ 9 -THC can be explained by weaker cannabinoid CB 1 receptor affinity, although there are other plausible mechanisms that may contribute. We highlight the gaps in our knowledge of Δ 8 -THC pharmacology where further studies are needed, particularly in humans.delta-8-THC, pharmacodynamics, pharmacokinetics, Δ 8 -THC | INTRODUCTIONDelta-8-tetrahydrocannabinol (Δ 8 -THC) is a cannabinoid that is a double bond isomer of the more well-known Δ 9 -THC (Figure 1). Δ 8 -THC was first derived from the cyclization of cannabidiol (CBD) and it was discovered to be highly psychoactive in human studies (Adams, 1942).By 1966, it was realized that Δ 8 -THC was present in only negligible amounts in cannabis and cannabis-derived products, such as hash (Hively et al., 1966). Δ 9 -THC was determined to be the compound, almost entirely, responsible for the intoxicating properties of cannabis, including alterations in mood, perception and cognition. Subsequent research focused much more on Δ 9 -THC, but the effects of Δ 8 -THC continued to be characterized throughout the following decades. One reason for this is the better thermodynamic stability of Δ 8 -THC relative to Δ 9 -THC (Hanuš et al., 2016). Note that early studies referred to Δ 8 -THC as Δ 6 or Δ 1(6) and Δ 9 -THC as Δ 1 . This review will follow the modern ring numbering system even when the original publication used the old numbering.There is currently debate about the regulatory status of Δ 8 -THC in the United States (e.g. Koski, 2021). The Agriculture Improvement Act of 2018 (informally called the 'Farm Bill') removed hemp and hemp products containing less than 0.3% Δ 9 -THC from the legal definition of marijuana in the Federal Controlled Substances Act (Agriculture Improvement Act of 2018, 2018). Importantly for Δ 8 -THC, hemp was defined as including 'all derivatives, extracts, cannabinoids, isomers, acids, salts and salts of isomers'. Because Δ 8 -THC is both an isomer of CBD and a derivative of CBD when obtained from the cyclization Abbreviations: C max , maximal concentration; CYP, cytochrome P450; P-gp, P-glycoprotein; Tmax, time to maximal concentration; Δ 8 -THC, delta-8-tetrahydrocannabinol.