From Desk Reference to the Diagnostic Criteria from DSM-3-R (American Psychiatric Association, 1987):
Caffeine-Induced Organic Mental Disorder 305.90 Caffeine Intoxication
- Recent consumption of caffeine, usually in excess of 250 mg.
- At least five of the following signs:
- flushed face
- gastrointestinal disturbance
- muscle twitching
- rambling flow of thought and speech
- tachycardia or cardiac arrhythmia
- periods of inexhaustibility
- psychomotor agitation
- Not due to any physical or other mental disorder, such as an Anxiety Disorder.
Basically, overdosing on caffeine will probably be very very unpleasant but not kill or deliver permanent damage. However, People do die from it.
The LD_50 of caffeine (that is the lethal dosage reported to kill 50% of the population) is estimated at 10 grams for oral administration. As it is usually the case, lethal dosage varies from individual to individual according to weight. Ingestion of 150mg/kg of caffeine seems to be the LD_50 for all people. That is, people weighting 50 kilos have an LD_50 of approx. 7.5 grams, people weighting 80 kilos have an LD_50 of about 12 grams.
In cups of coffee the LD_50 varies from 50 to 200 cups of coffee or about 50 vivarins (200mg each).
One exceptional case documents survival after ingesting 24 grams. The minimum lethal dose ever reported was 3.2 grams intravenously, this does not represent the oral MLD (minimum lethal dose).
In small children ingestion of 35 mg/kg can lead to moderate toxicity. The amount of caffeine in an average cup of coffee is 50 - 200 mg. Infants metabolize caffeine very slowly.
From conversations on alt.drugs.caffeine:
The toxic dose is going to vary from person to person, depending primarily on built-up tolerance. A couple people report swallowing 10 to 13 vivarin and ending up in the hospital with their stomaches pumped, while a few say they've taken that many and barely stayed awake.
A symptom lacking in the clinical manual but reported by at least two people on the net is a loss of motor ability: inability to move, speak, or even blink. The experience is consistently described as very unpleasant and not fun at all, even by those very familiar with caffeine nausea and headaches.