Short answers, pulled from the story.
Caffeine is a central nervous system stimulant of the methylxanthine class and is the most widely consumed psychoactive substance in the world. It works by blocking adenosine receptors in the brain, preventing the accumulation of adenosine from triggering drowsiness and thereby maintaining alertness. Global consumption has been estimated at 120,000 tonnes per year.
The European Food Safety Authority has found that up to 400 mg per day does not raise safety concerns for healthy, non-pregnant adults. For pregnant women, both the American Congress of Obstetricians and Gynecologists and the UK Food Standards Agency recommend limiting intake to 200 mg per day. Health Canada recommends no more than 400 mg per day for non-pregnant adults and no more than 300 mg per day for women who are pregnant or breastfeeding.
Caffeine withdrawal symptoms include headache, fatigue, difficulty concentrating, depressed mood or irritability, and flu-like symptoms. These symptoms must appear within 24 hours of a marked reduction in caffeine intake following prolonged daily use. The ICD-11 includes caffeine dependence as a distinct diagnostic category.
Caffeine was first isolated in 1819 by German chemist Friedlieb Ferdinand Runge, who called it Kaffebase. In 1821, it was independently isolated by French chemist Pierre Jean Robiquet and by the pair Pierre-Joseph Pelletier and Joseph Bienaimé Caventou. The total synthesis of caffeine from its chemical components was first achieved in 1895 by German chemist Hermann Emil Fischer, whose broader work earned him the Nobel Prize in 1902.
The lethal dose of caffeine in humans is estimated at 150-200 milligrams per kilogram of body weight, equivalent to roughly 10.5-14 grams for a typical 70 kg adult. This corresponds to approximately 75-100 ordinary cups of coffee. Powdered caffeine supplements carry a much higher risk because the estimated lethal amount is less than a tablespoon.
Caffeine is a proven ergogenic aid in humans. Moderate doses of around 5 mg per kilogram of body weight have been shown to improve sprint performance, cycling and running time trials, endurance, and cycling power output. Caffeine reduces perceived exertion and delays the onset of both muscle fatigue and central fatigue.