Caffeine is a stimulant, and an overdose can be lethal. As for other side effects, caffeine is similar to other stimulants. If you do not regularly consume caffeine, I would avoid taking more than 2 cups in any 4 hour period.
The median lethal dose of caffeine is anywhere between 150 and 200 milligrams per kg of body weight in a human. A cup of coffee has around 100 milligrams of caffeine.
So, you would need around 2x your bodyweight in kilograms of cups of coffee to kill yourself. There have been no known instances of lethal caffeine overdoses from drinking coffee alone. Probably because drinking that much coffee would be impractical for even the most dedicated person.
Now, even at lower doses, there can be severe adverse side effects, and you’re probably also interested to know this.
Overdosing on caffeine can result in palpitations, twitching, and nervousness as well as increasing your heart rate. But since caffeine is a drug easily adapted to, its hard to determine what dosage would result in significant side effects for a given individual.
Caffeine operates by binding to the adenosine receptors without activating them, stopping adenosine from activating them. Adenosine has an inhibitory effect on the central nervous system. Taking caffeine regularly will cause your body to increase the number of adenosine receptors – thus blunting its effect.