The symptoms caffeine poisoning are restlessness, nervousness,excitement, insomnia, flushed face, diuresis, gastrointestinal complaints, even hallucinations. They can occur in some people after as little as 250 mg per day. More than 1 g per day may result in muscle twitching, rambling flow of thought and speech, cardiac arrhythmia or tachycardia, and psychomotor agitation. Caffeine intoxication can lead to symptoms similar to panic disorder and generalized anxiety disorder.

The minimum lethal dose ever reported was 3.2 g, intravenously. The LD50 of caffeine (that is the lethal dosage reported to kill 50% of the population) is estimated between 13-19 grams for oral administration for a average adult. The LD50 of caffeine is dependent on weight and estimated to be about 150-200 mg per kg of body mass, roughly 140-180 cups of coffee for an average adult taken within a limited timeframe that is dependent on half life. The half-life or time it takes to metabolize 50% of the caffeine, ranges from 3.5 to 100 hours. In adults the half-life is generally around 5 hours. However contraceptive pills increase this to around 12 hours and for women over 3 months pregnant it varies from 10 to 18 hours. In infants and young child the half-life may be longer than adults. With common coffee and a very rare half-life of 100 hours it would require 3 cups of coffee every hour for 100 hour just to reach LD50. Though achieving lethal dose with coffee would be exceptionally difficult, there have been many reported deaths from intentional overdosing on caffeine pills.