Some people diagnosed with major depressive disorder (MDD) are poor or non-responders to widely used antidepressant medica- tion, such as selective serotonin reuptake inhibitors (SSRIs), serotonin- norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs) (Henkel et al., 2009). These drugs are often not fast-acting, having a typical time lag of 2–4 weeks initially, and 6–12 weeks for maximum effect (Niciu et al., 2015).