The disclosure provides crystalline and amorphous forms of aticaprant. Also provided by the disclosure are pharmaceutical compositions comprising the amorphous or crystalline forms, methods of treating major depressive disorder using the amorphous or crystalline forms of aticaprant, amorphous or crystalline forms of aticaprant for use in the treatment of major depressive disorder in a human patient having anhedonia, uses of the amorphous or crystalline forms of aticaprant in the manufacture of a medicament for the treatment of major depressive disorder, and packages or pharmaceutical products comprising (i) amorphous or crystalline forms of aticaprant and (ii) instructions for treating major depressive disorder. In some aspects, the human patient treated as described herein has anhedonia.