Theine, contained in tea, is a poisonous substance similar to the caffeine of coffee. Caffeine and theine are similar in their effects upon the body, producing "an increased metabolism or energy production (expenditure) ; an increased rate of respiration, a considerable diuresis (urination), and muscular and nervous stimulation" (irritation). The nervous and muscular irritation lead to over activity and fatigue, to nervous instability or hyper-irritability and to loss of sleep. The use of these substances leads to exhaustion through overexcitation.

Underweight, probably due to nervous and digestive impairment, is repeatedly seen, in health surveys, to be more common in coffee and tea drinkers than in those who do not employ these poisons.