Excessive caffeine intake can lead to a fast heart rate, diuresis (excessive urination), nausea and vomiting, restlessness, anxiety, depression, tremors, and difficulty sleeping.

The effect of caffeine on health has been widely studied. In particular, the effects of caffeine on fibrocystic breast disease, cardiovascular disease, birth defects, reproductive function, and behavior in children has been closely examined and linked to some but not all.

A 1984 statement from the American Medical Association Council on Scientific Affairs stated "Moderate tea or coffee drinkers probably have no concern for their health relative to their caffeine consumption provided other lifestyle habits (diet, alcohol consumption) are moderate as well." This statement emphasizes moderate caffeine use.

Abrupt withdrawal of caffeine may cause headaches, drowsiness, irritability, nausea, vomiting, and other symptoms. Reduce caffeine intake gradually to prevent any symptoms of withdrawal.