Cannabis elicits its mood-enhancing and analgesic effects through the cannabinoid receptor 1 (CB1), a G protein-coupled receptor (GPCR) that signals primarily through the adenylyl cyclase-inhibiting heterotrimeric G protein Gi. Activation of CB1-Gi signaling pathways holds potential for treating a number of neurological disorders and is thus crucial to understand the mechanism of Gi activation by CB1. Here, we present the structure of the CB1-Gi signaling complex bound to the highly potent agonist MDMB-Fubinaca (FUB), a recently emerged illicit synthetic cannabinoid infused in street drugs that have been associated with numerous overdoses and fatalities. The structure illustrates how FUB stabilizes the receptor in an active state to facilitate nucleotide exchange in Gi. The results compose the structural framework to explain CB1 activation by different classes of ligands and provide insights into the G protein coupling and selectivity mechanisms adopted by the receptor. Display omitted •3 Å cryo-EM structure of the CB1-Gi complex bound to potent agonist MDMB-Fubinaca•MDMB-Fubinaca locks “toggle switch” residues F2003.36/W3566.48 in active conformation•Quantum mechanics calculations reveal the mechanism for the high affinity of Fubinaca•Molecular dynamic simulations reveal a path for ligand entry between TM1 and TM7 Looking at how a toxic, synthetic ligand locks cannabinoid receptor 1 into a signaling conformation points to ways to understand and modulate receptor activity.