Crystalline N-methyl tryptamine derivatives, compositions containing those crystalline forms and their methods of use are disclosed. The crystalline N-methyl tryptamine derivatives according to the invention include crystalline N-methyl-N-propyltryptamine (MPT), crystalline N-methyl-N-isopropyltryptammonium fumarate (MiPT fumarate) and crystalline 4-hydroxy-N-methyl-N-isopropyltryptammonium fumarate monohydrate (HO-MiPT fumarate monohydrate).