There is provided a compound of formula I, wherein L1, R1, R2, R5, X, A and B have meanings given in the description, and pharmaceutically acceptable salts, solvates and prodrugs thereof, which compounds are useful as antagonists of the orexin-1 and orexin-2 receptors or as selective antagonists of the orexin-1 receptor, and thus, in particular, in the treatment or prevention of inter alia substance dependence, addiction, anxiety disorders, panic disorders, binge eating, compulsive disorders, impulse control disorders, cognitive impairment and Alzheimer' s disease.