Anxiety

Caffeine is known to make people more alert. It works by blocking adenosine's effects, a brain substance that causes fatigue. Adrenaline, the "fight-or-flight" hormone linked to increased energy, is produced at the same time.

However, with greater dosages, these effects could be more noticeable, resulting in anxiety and nervousness. In fact, the Diagnostic and Statistical Manual of Mental Disorders (DSM), a publication of the American Psychiatric Association, lists four caffeine-related disorders, with caffeine-induced anxiety disorder being one of them. Most people have been known to suffer anxiety, jitteriness, and similar symptoms at extremely high daily doses of 1,000 mg or more, although caffeine-sensitive people may have the same symptoms with even a small consumption.