The future of type 1 cannabinoid receptor allosteric ligands. 
Allosteric modulation of the type 1 cannabinoid receptor (CB1R) holds great therapeutic potential.  This is because allosteric modulators do not possess intrinsic efficacy, but instead augment (positive allosteric modulation) or diminish (negative allosteric modulation) the receptor's response to endogenous ligand.  Consequently, CB1R allosteric modulators have an effect ceiling which allows for the tempering of CB1R signaling without the desensitization, tolerance, dependence, and psychoactivity associated with orthosteric compounds.  Pain, movement disorders, epilepsy, obesity are all potential therapeutic targets for CB1R allosteric modulation.  Several challenges exist for the development of CB1R allosteric modulators, such as receptor subtype specificity, translation to in vivo systems, and mixed allosteric/agonist/inverse agonist activity.  Despite these challenges, elucidation of crystal structures of CB1R and compound design based on structure-activity relationships will advance the field.  In this review, we will cover recent progress for CB1R allosteric modulators and discuss the future promise of this research.