Green Tea : Safety

by MedlinePlus

(Page 2 of 3)

Dosing

Benefits of specific doses of green tea are not established. Most studies have examined green tea in the form of a brewed beverage, rather than in capsule form. One cup of tea contains approximately 50 milligrams of caffeine and 80 to 100 milligrams of polyphenol content, depending on the strength and size of cup. For cancer prevention, studies have examined the effects of habitually drinking anywhere from 1 to 10 cups per day (or greater). Preliminary data shows that drinking 4 cups/day of decaffeinated green or black tea may decrease the amount of DNA damage in heavy smokers. For heart disease prevention, one study reports benefits of drinking greater than 375 milliliters of tea per day, although evidence in this area remains unclear.

In capsule form, there is considerable variation in the amount of green tea extract (GTE) per capsule, anywhere from 100 to 750 milligrams per capsule. Extracts of green tea may be standardized to contain anywhere from 60 to 97 percent polyphenols. A 2001 phase I study by Pisters et al. reported that the maximum-tolerated dose of oral green tea extract (GTE) is 4.2 grams per meter squared daily taken daily once or in three divided doses (equivalent to 7 or 8 Japanese cups), or 120 milliliters of green tea three times daily. Another phase I study in 2005 by Laurie et al. reported that the maximum-tolerated dose of green tea extract (GTE) in patients with advanced lung cancer was 3g/m2per day. Currently, there is no established recommended dose for GTE capsules.

Safety

Allergies

People with known allergy/hypersensitivity to caffeine or tannin should avoid green tea. Skin rash and hives have been reported with caffeine ingestion.

Side Effects and Warnings

Studies of the side effects of green tea specifically are limited. However, green tea is a source of caffeine, for which multiple reactions are reported.

Caffeine is a stimulant of the central nervous system, and may cause insomnia in adults, children, and infants (including nursing infants of mothers taking caffeine). Caffeine acts on the kidneys as a diuretic (increasing urine and urine sodium/potassium levels, and potentially decreasing blood sodium/potassium levels), and may worsen urge incontinence. Caffeine-containing beverages may increase the production of stomach acid, and may worsen ulcer symptoms. Tannin in tea can cause constipation. Caffeine in doses of 250 to 350 milligrams can increase heart rate and blood pressure, although people who consume caffeine regularly do not seem to experience these effects in the long-term.

An increase in blood sugar levels may occur after drinking green tea containing the equivalent of 200 milligrams of caffeine (4 to 5 cups, depending on tea strength and cup size). Caffeine-containing beverages such as green tea should be used cautiously in patients with diabetes. In contrast, lowering of blood sugar levels from drinking green tea has also been reported in preliminary research. Additional study is needed in this area.

People with severe liver disease should use caffeine cautiously, as levels of caffeine in the blood may build up and last longer. Skin rashes have been associated with caffeine ingestion. In laboratory and animal studies, caffeine has been found to affect blood clotting, although effects in humans are not known.

Caffeine toxicity/high doses: When the equivalent or more than 500 milligrams of caffeine are consumed (usually more than 8 to 10 cups per day, depending on strength and size of cups), symptoms of anxiety, delirium, agitation, psychosis, or detrussor instability (unstable bladder) may occur. Conception may be delayed in women who consume large amounts of caffeine. Seizure, muscle spasm, life-threatening muscle breakdown (rhabdomyolysis), and life-threatening abnormal heart rhythms have been reported with caffeine overdose. Doses greater than 1000 milligrams may be fatal.

Caffeine withdrawal: Chronic use can result in tolerance, psychological dependence, and may be habit forming. Abrupt discontinuation may result in withdrawal symptoms such as headache, irritation, nervousness, anxiety, tremor, or dizziness. In people with psychiatric disorders such as affective disorder or schizoaffective disorder, caffeine withdrawal may worsen symptoms or cause confusion, disorientation, excitement, restlessness, violent behavior, or mania.

Chronic effects: Several population studies initially suggested a possible association between caffeine use and fibrocystic breast disease, although more recent research has not found this connection. Limited research reports a possible relationship between caffeine use and multiple sclerosis, although evidence is not definitive in this area. Animal study reports that tannin fractions from tea plants may increase the risk of cancer, although it is not clear that the tannin present in green tea has significant carcinogenic effects in humans.

Drinking tannin-containing beverages such as tea may contribute to iron deficiency, and in infants, tea has been associated with impaired iron metabolism and microcytic anemia.

In preliminary research, green tea has been associated with decreased levels of estrogens in the body. It is not clear if significant side effects such as hot flashes may occur.

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