Alzheimer’s patients look thin. In fact, it may not be thin; it is caused by long-term chronic dehydration. It has been known that chronic dehydration causes not only serum hyperosmotic pressure increase but also dysfunction in water, salt, and electrolyte metabolism, as well as acid-base balance in the central nervous system (CNS), resulting in cognitive impairment. Recently, an imbalance in the anabolism and catabolism of endogenous formaldehyde was considered as one of the risk factors involving age-related cognitive impairment. The levels of endogenous formaldehyde in inpatients were markedly higher than those in the age-matched participants. Thus, we are concerned about the relationship between cognitive impairment and water intake behavior, as well as that between dehydration and dysmetabolism of endogenous formaldehyde with aging. As a result of aging, serum arginine vasopressin (AVP) concentration and brain endogenous formaldehyde levels increase. Underwater deprivation, formaldehyde increased in the brains of C57BL/6j mice. An increase in urine formaldehyde was observed by water deprivation of university students from morning to lunchtime (5–6 h). Excess formaldehyde could activate ANG II to promote AVP expression. Meanwhile, the elevation of AVP activates semicarbazide-sensitive amine oxidase (SSAO) to produce more endogenous formaldehyde. Chronic hypertonic dehydration affecting learning with emotion may result from decreased serotonin by SSAO deamination in the mouse brain. These pathways could create a vicious cycle that worsens drinking behavior and reduces the frequency and amount of water intake. That is, at least, dysregulation of endogenous formaldehyde can affect water intake behavior. The highest concentration of endogenous formaldehyde is found in the morning, followed by a decrease around noon and an increase in the evening. Therefore, regular water intake including tea is recommended to intervene and reduce formaldehyde accumulation in the human body, especially when it is taken in the morning.