Because the brain cells shrink with age, the weight of the brain decreases. On average, when a person is 70, his brain weight is 5 % lower than when they he was 30. The figures are 10 % for 80 years old, and 20 % for 90. But the body weight doesn't necessarily decrease.
由於人體腦細胞也會隨老化而萎縮，使腦的重量減少，但全身重量卻不見得減少，因此如果依照傳統上「按體重計算用藥劑量」的方式，往往會造成過量，特別是以治療腦神經血管疾病的各種抗精神病藥物，最容易有這方面的問題。