The content of methylxanthines (caffeine, theobromine and theophylline) varies depending on the type of tea, the botanical variety, the temperature of the water used for the infusion and the duration of the infusion itself.
On average, black tea contains mainly caffeine, while the other two xanthines are contained in greater quantities in green teas etc.
As compared with coffee, tea contains less caffeine: a cup of around 150 cc of strong black tea contains 40 mg of caffeine and a cup of green tea contains 20 mg, as against the 100-125 mg of a 40 cc cup of coffee.
The principal effect attributed to caffeine is its action as a stimulant to human mental faculties, while it also reduces the impression of fatigue. In addition, it increases renal circulation and the number of glomeruli, thus stimulating diuresis.
Normally an adult can consume up to 400 mg of caffeine a day without any adverse effects such as general toxicity, cardiovascular effects and mood swings.