Caffeine overdoses really profoundly suck. I have done this to my body only once, and I hope never to repeat the experience. Follow my example, and you'll discover a whole new, unique, world of
shityness.
Caffeine's LD50 is somewhere around 10g, however, its rather harmful, toxic, and generally really bad for you at much lower doses. From my experience, the amount of caffeine it takes for things to become seriously unpleasant is dependent upon a variety of factors including caffeine tolerance, hydration, sleep, activity, and the phase of the moon. However, it's generally a good rule of thumb to never exceed about 1g a day. Personally, I've sworn never to take more than 600mg per day, due to this experience.
Anyhow, when I overdosed on caffeine, it was two years ago during my freshman year of high school, which was generally the most unpleasant year of my life. That year, I had what could be called a caffeine problem, which seriously managed to mess up my body and make me feel like shit most of the time. A normal weekday would follow the following pattern: two mugs of coffee in the morning, one 20oz mountain dew at break, another 20oz mountain dew at lunch, and maybe a frapuccino at home, totaling somewhere in the neighborhood of 400mg of caffeine. And I still crashed all afternoon. Because of this, I had a pretty phenomenal caffeine tolerance. I couldn't notice the effects of anything less than maybe 100mg, if not more. I'd also taken up the practice of drinking/ingesting copious quantities of caffeine, usually around 600mg, at night during the weekend, usually with friends, where I'd play video games and stuff like that.
The day I overdosed on caffeine, I had recently (that day? I don't remember) gotten a package of 200mg NoDoz caffeine pills. Anyhow, I was swallowing those things like candy all day. A pill in the morning, crush a pill into a smoothie (note: when crushing caffeine pills into smoothies, using your fingers is not adequate. It must be reduced to a fine powder, otherwise you get icky bitter caffeine pill chunks in your drink), whatever. I am not sure how much I had during the day, a random guess is somewhere between 400-600mg. Anyhow, that evening a friend and myself were at my place, and despite my stimulant loaded day, I was getting tired. So I popped a pill. And another. And still, I felt pretty tired. I never did get that typical bouncy-fun feeling that I usually would. So another, and another. I had between 1-1.2g of caffeine that night.
Let me tell you something, if you take up wards of 500mg of caffeine, and you're not bouncing off of things, it's just not gonna work, and you should just go the fuck to sleep, like your body obviously wants you to. Anyhow, the feelings that resulted that evening were really profoundly unpleasant. For most of the night, I felt twitchy, and nauseous, my head felt like someone had fired a .50 caliber Desert Eagle of pain into my forehead, I was sweating and dehydrated, and I was more or less curled up into a fetal ball of delusional feverish pain. It is an exceptionally unusual feeling being both tweaked out and twitchy while at the same time totally wasted. I must've thrown up a dozen times. This pretty much lasted from 10:00pm to 8:00am, when I finally managed to crash into a restless sleep on the couch in the living room (I don't remember why I didn't crash in my room, I think that my friend was making some degree of noise which to my fucked brain was like hammering my temples with a sledgehammer).
But that wasn't an end to the fun. Then came-- the hangover! Now, to be entirely honest, I've never been particularly drunk. In fact, at that time, I don't think I'd ever been inebriated to any extent. While I've certainly not had a hang over from alcohol before, judging from seeing other people, this was pretty fucking bad. I had a pounding headache, and threw up intermittently until the next day.
So, yeah, the moral of this story is that don't take a whole lot of caffeine, because it really fucking sucks.