Green tea is LIKELY SAFE for most adults. Green tea extract is POSSIBLY SAFE for most people for short-term use. In some people, green tea can cause stomach upset and constipation. Green tea extracts have been reported to cause liver problems in rare cases.
Too much green tea — more than five cups per day, for example — is POSSIBLY UNSAFE. It can cause side effects because of the caffeine. These side effects can range from mild to serious and include headache, nervousness, sleep problems, vomiting, diarrhea, irritability, irregular heartbeat, tremor, heartburn, dizziness, ringing in the ears, convulsions, and confusion. Green tea seems to reduce the absorption of iron from food. Drinking very high doses of green tea can actually be fatal. The fatal dose of caffeine in green tea is estimated to be 10-14 grams (150-200 mg per kilogram). Serious toxicity can occur at lower doses.
Caffeine is POSSIBLY SAFE in children in amounts commonly found in foods.
Green tea interacts with many medications, as explained below.
Special Precautions & Warnings:
Pregnancy and breast-feeding: If you are pregnant or breast-feeding, green tea in small amounts is POSSIBLY SAFE. Do not drink more than 2 cups a day of green tea. This amount of tea provides about 200 mg of caffeine. Consuming more than this amount has been linked to an increased risk of miscarriage and other negative effects. Caffeine passes into breast milk and can affect a nursing infant. Don’t drink an excessive amount of green tea if you are breast-feeding.
“Tired blood” (anemia): Drinking green tea may make anemia worse.
Anxiety disorders: The caffeine in green tea might make anxiety worse.
Bleeding disorders: Caffeine might increase the risk of bleeding. Don’t drink green tea if you have a bleeding disorder.
Heart conditions: Caffeine in green tea might cause irregular heartbeat.
Diabetes: Caffeine might affect blood sugar control. If you drink green tea and have diabetes, monitor your blood sugar carefully.
Diarrhea. Green tea contains caffeine. The caffeine in green tea, especially when taken in large amounts, can worsen diarrhea.
Irritable bowel syndrome (IBS): Green tea contains caffeine. The caffeine in green tea, especially when taken in large amounts, can worsen diarrhea and might worsen symptoms of IBS.
Glaucoma: Drinking green tea increases pressure inside the eye. The increase occurs within 30 minutes and lasts for at least 90 minutes.
High blood pressure: The caffeine in green tea might increase blood pressure in people with high blood pressure. However, this does not seem to occur in people who regularly drink green tea or other products that contain caffeine.
Liver disease: Green tea extract supplements have been linked to several cases of liver damage. Green tea extracts might make liver disease worse.
Weak bones (osteoporosis): Drinking green tea can increase the amount of calcium that is flushed out in the urine. Caffeine should be limited to less than 300 mg per day (approximately 2-3 cups of green tea). It is possible to make up for some calcium loss caused by caffeine by taking calcium supplements.