suicide reported in this meta-analysis may be explained by the confounding effects of these well-known risk factors. However, another part of the association may be the result of the effect of smoking itself that may increase the risk of suicide through a biological pathway. It is suggested that smoking can significantly decrease the activity of the serotonergic system of the human hippocampus and may reduce brain serotonin function which is negatively related to risk of suicide [89, 90]. Furthermore, nicotine is a potent activator of the hypothalamus, pituitary, adrenal (HPA) axis and is able to activate the attenuated responsiveness of the HPA axis to psychological stress. On the other hand, hyperactivity of the HPA axis is supposed to be a risk factor for suicidal behaviors [91, 92].
