Metabolites of all plant species can be divided into universal and special metabolites. In a chemotaxonomic view, cacti also synthesize and contain certain special metabolites. Most of the cacti contain a large amount of mucilage, which compounds are important in the retention of water under drought conditions. Certain species such as that of the Mammillaria genus (subgenus Mammillaria and partially Subhydrochylus) contain cactus latex instead of mucilage. Despite the different appearance, the chemical composition of cactus latex and mucilage are similar. Some species accumulate calcium-oxalate crystals in their body. Among the nitrogen-containing special metabolites, several cactus taxon synthesize alkaloids of tyrosine or histidine origin. Betalains are tyrosine alkaloids which function in excess light protection and antimicrobial activity in cacti. Among the psychoactive compounds, several taxa contain tyramine alkaloids such as condicine, N-methyltyramine or mescaline. In the tribus Pachycereae, the tetrahydro-isoquinoline alkaloids are common. In certain Mammillaria and Echinocereus taxa, imidazole alkaloids of histidine origin can be found. Cacti synthesise these alkaloids to defend themselves against microbial and animal pests.