A University of Melbourne study called caffeine "the most commonly used psychoactive drug," and said drinking more than 5 cups a day can have you hearing things.
From A 2011 story published in The Daily Mail:
Professor Simon Crowe, of La Trobe University in Melbourne, said: "We also told them that within the white noise there may be parts of the song White Christmas and if you hear it, press a button. We didn’t include White Christmas in the white noise but found that more people who were very stressed and had high levels of caffeine thought they heard the song. The combination of caffeine and stress affect the likelihood of an individual experiencing a psychosis-like symptom."
High levels of caffeine can damage your liver, especially if you take pain medication regularly as well.
According to studies by the National Institute of Diabetes and Digestive and Kidney Diseases.
It can raise your blood pressure.
That's according to a 2012 Carnegie Mellon study which also says that coffee can stimulate the heart and cause shallow breathing.
It can make you irrational and irritable by stymieing oxygen's flow to the brain.
It can cause insomnia.
It's addictive and can cause withdrawal, according to John's Hopkins.
Withdrawal when coming off caffeine can give you a headache.
Withdrawal hits about 12-24 hours after your last cup, according to the American Heart Association. Not only are you tired and have lowered performance, but you can get a headache as well.
Withdrawal can also make you depressed. And you can add anxiety, fatigue, and drowsiness to those symptoms.
It can make hard workers slack off says one University of British Columbia study.
The University of British Columbia did a study in which it gave 40 rats amphetamines and caffeine. The amphetamines made lazy rats work harder and hard working rats more lazy.
The coffee, on the other hand, did nothing for the slackers. It didn't do anything for the workers either.
It can increase your risk for osteoporosis, so make sure you're getting enough calcium and vitamin D.
BONUS: Drinking caffeine can lead to a leakier bladder according to the University of Alabama
Source : http://www.businessinsider.com/10-reasons-not-to-drink-too-much-coffee-2013-8?op=1