This in vitro study examined the inhibitory effects of five structural analogues of methylene blue (MB)—neutral red, Nile blue, new methylene blue, cresyl violet, and 1,9-dimethyl methylene blue—on human monoamine oxidase (MAO) enzymes. The results demonstrated that all analogues selectively inhibited the MAO-A isoform, with cresyl violet (IC₅₀ = 0.0037
