Caffeine.

Caffeine/coffee is likely safe for most healthy adults.

The most people should be consuming per day is 400mg, which is approximately 4 cups per day. Anything more than this can cause side effects, such as... Insomnia, nervousness, restlessness, increased heart rate, and nausea.

Another effect of using high amounts of caffeine is Adrenal fatigue!

Caffeine stimulates neuron activity in the brain in which the neurons send messages to the pituitary glands which produce adrenaline and cortisol, these hormones cause the Fight or flight mode in humans. So when the pituitary glands are forced to crank these hormones out all the time, it will cause the glands to operate less efficiently, which in turn can cause us to become more tired, and require more and more stimulants to maintain alertness. But using more of this stimulant can also cause the other symptoms to exacerbate.