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Chronic stress can also lead to the disruption of the Blood-Brain Barrier (BBB). The brain is unique among all other organs, for it has a special membrane structure that blocks the majority of chemicals from entering the brain through the blood. However, the brain is particularly susceptible to chronic stress, especially in early childhood and old age. When under conditions of stress for a long period of time, stress may transiently open the Blood-Brain Barrier and therefore let toxins, drugs or other unfavorable chemicals into the brain.