Overview Caffeine is a bitter-tasting stimulant that is an ingredient in certain beverages, including coffee and green tea. Caffeinated beverages can provide a burst of energy, but should only be used in moderation. According to Medline Plus, too much caffeine can cause irritability, anxiety and restlessness. To monitor caffeine intake, you should know the amounts found in green tea and coffee.Types On average, a cup of coffee contains about 115 mg of caffeine when prepared by the drip method, according to the International Coffee Organization. When ground coffee is prepared using a percolator, caffeine amounts drop to an average of 80 mg. Instant coffees contain an average of 65 mg of caffeine. These amounts are based on a 5-oz. serving size.Features According to the Medicinal Food News website, a 150 ml (5 oz.) serving of green tea contains approximately 48 mg of caffeine. The way that you brew the green tea can affect caffeine amounts. The longer that you leave the tea leaves in the boiling water, the higher the caffeine amount will be. If the same green tea leaves are used multiple times, caffeine amounts drop with each infusion.Identification There are decaffeinated versions of coffee and green tea. To be labeled as decaffeinated, the products must contain less than 0.1 percent caffeine per the U.S. Department of Agriculture's regulations.Considerations According to the International Coffee Organization, although teas in general contain less caffeine than coffees, a strong brewed tea may have more caffeine than a cup of weak coffee. In addition, loose tea contains less caffeine than tea bags, while robusta coffees have nearly twice as much caffeine as arabicas.Warning If you have a sensitivity to caffeine, avoid coffee and green tea. Doctors advise patients with gastroesophageal reflux disease (GERD) to eliminate coffee from their diet, as coffee may increase reflux and cause symptoms such as heartburn; green tea doesn't seem to have the same effects. Pregnant or nursing women are also typically advised by their doctors to avoid caffeinated beverages.