However, it is essential to consider that its effects, as well as the effect of the pharmacological treatment, can be modified by genetic polymorphisms of BDNF and monoamines [e.g., monoamine oxidase (MAO), and to catechol-O-methyltransferase (COMT)] and several other factors such as sex, age, central nervous disease, medicines with effect in the central nervous system, etc. Also, tDCS effects are likely to be neuroplasticity state dependent, since previous study found that serum BDNF predicts the impact of tDCS on behavioral measures in chronic pain conditions (Souza et al., 2018). Here, COMT is linked to central nervous system disorder.