Methylation is a rare metabolic pathway in drug metabolism, but it is very important for the metabolism of some endogenous substances such as adrenaline and melatonin. The groups involved in methylation reaction include phenolic hydroxyl group, amino group, sulfhydryl group and so on. The main targets of phenolic hydroxyl methylation are active substances with catecholamine structure, such as epinephrine, norepinephrine and dopamine. In addition, the methylation reaction is regioselective and only occurs on the 3- position phenolic hydroxyl group. For example, adrenaline methylation produces 3-O- methyl adrenaline.