Any substance, even water, can be toxic in large amounts.
Caffeine, like cocaine, is an analeptic- a central nervous system stimulator. However it is considered a low grade drug and does have legitimate medical uses, such as in tension headaches and athletics. Caffeine does cause dependency, and the DSM considers 9-30% of of users may be dependent- it also causes withdrawl in heavy users. It can cause intoxication, whose symptoms resemble a panic attack. It can also cause anxiety and sleep deprivation.
Symptoms of caffeine intoxication include restlessness, diuresis, flushed face, insomnia, muscle twitches, diarrhea, and even tachycardia.
it has been linked to birth defects if consistently consumed in amounts over 200mg per day by pregnant women.
Caffeine overworks the nervous system. Excitement does the same. Excitement tends to make us tired so when we stop being excited we rest. With caffeine we can't stop until it is eliminated by the body so it makes us exhausted. Continuous exhaustion wears out the body because there is no way to rest and rejuvenate.
Here is a link from the Mayo Clinic regarding caffeine use. Caffeine has many benefits, but some people are more sensitive to the negative effects and those people should avoid large doses or even all caffeine. http://www.mayoclinic.com/health/caffeine/NU00600