Though caffeine is technically a “drug,” its effects are relatively benign. Caffeine will begin to affect the body at a concentration of 15 milligrams per liter (mg/L) in the blood. Most of the time the effects do not cause lasting harm.
However, while a fatal or even life-threatening overdose of caffeine is quite rare, consuming very large amounts of caffeine can be harmful. A concentration of 80 to 100 mg/L of caffeine in the body can even be fatal.
Read more here: https://www.answeraddiction.com/is-it-possible-to-overdose-on-caffeine/
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