CAFFEINE EXCESS
For most people, moderate doses of caffeine — 200 to 300 milligrams (mg) or about two to three cups of brewed coffee a day— aren't harmful. Though moderate caffeine intake isn't likely to cause harm, too much can noticeably affect your health. Some circumstances, such as caffeine sensitivity or use of certain medications, may warrant limiting or even ending your caffeine routine. Heavy daily caffeine use — more than 500 to 600 mg a day, or about four to seven cups of coffee — can cause the following symptoms: restlessness; anxiety; irritability; muscle tremors; sleeplessness; headaches; nausea, diarrhea or other gastrointestinal problems; and abnormal heart rhythms. You should check with your doctor immediately if you experience any of the symptoms after drinking coffee or foods containing caffeine.
(Source: Mayo Foundation for Medical Education and Research 2007)
See also:
Caffeine
