Normally, almost all ingested caffeine is metabolized. Short-term effects of a drug are those that appear soon after a single dose and disappear within hours. General metabolism increases - expressed as an increase in activity or raised temperature, or both. The rate of breathing increases, as does urination and the levels of fatty acids in the blood and of gastric acid in the stomach. Caffeine use may increase blood pressure.
Caffeine stimulates the brain and behavior.
Use of 75-150 mg of caffeine elevates neural activity in many parts of the brain, postpones fatigue, and enhances performance at simple intellectual tasks and at physical work that involves endurance but not fine motor coordination. Caffeine's effects on complex intellectual tasks and on mood do not lend themselves to a simple summary. The effects depend on the personality of the user, on the immediate environment, on the user's knowing whether caffeine has been taken, and even on the time of day. After using caffeine, sleepers are more easily aroused, move more during sleep, and report a reduction in the quality of sleep. The effects of caffeine on dreaming are less clear.