Black tea is made from the fermented leaves of the Camellia sinensis plant. These dried leaves and leaf buds are also used to make green and oolong teas.

Black tea contains 2% to 4% caffeine, which affects thinking and alertness. It also contains antioxidants and other substances that might help protect the heart and blood vessels. It has different properties than green tea, which is made from the fresh leaves of the same plant.

People use black tea for improving mental alertness. It is also used for headache, both low and high blood pressure, preventing Parkinson disease, depression, dementia, stroke, and many other conditions, but there is no good scientific evidence to support many of these uses.

Don’t confuse black tea with other caffeine sources, including coffee, oolong tea, and green tea. These are not the same.

Likely Effective for

• Mental alertness. Drinking black tea and other caffeinated beverages helps to keep people alert and improve attention, even after extended periods without sleep.

Possibly Effective for

• Low blood pressure. Drinking caffeinated beverages, including black tea, helps increase blood pressure in older people who have low blood pressure after eating.
• Heart attack. People who drink black tea seem to have a lower risk of having a heart attack.
• Weak and brittle bones (osteoporosis). Regularly drinking black tea seems to improve bone health by a small amount.
• Ovarian cancer. People who regularly drink tea, including black tea or green tea, seem to have a lower risk of developing ovarian cancer compared to those who never or rarely drink tea.
• Parkinson disease. People who drink caffeinated beverages, including black tea, seem to have a lower risk of Parkinson disease.

Possibly Ineffective for

• Bladder cancer. People who drink tea, including black tea and green tea, do not seem to have a lower risk of bladder cancer compared to those who don’t drink tea.
• Breast cancer. Drinking black tea isn’t linked to a lower risk of developing breast cancer.
• Colon cancer, rectal cancer. Drinking black tea isn’t linked with a lower risk of colon and rectal cancer.
• Diabetes. Taking an extract of black and green tea by mouth doesn’t improve blood sugar levels in people with diabetes. Also, drinking black tea isn’t linked with a lower risk of developing diabetes.
• Cancer of the lining of the uterus (endometrial cancer). Drinking black tea isn’t linked to a lower risk of developing endometrial cancer.
• Cancer of the esophagus. Drinking black tea isn’t linked to a lower risk of developing cancer of the esophagus.
• Stomach cancer. Drinking black tea isn’t linked to a lower risk of developing stomach cancer.
• Lung cancer. Drinking black tea isn’t linked to a lower risk of developing lung cancer.

There is interest in using black tea for a number of other purposes, but there isn’t enough reliable information to say whether it might be helpful.

When taken by mouth: Black tea is commonly consumed as a beverage. Drinking black tea in moderate amounts (about 4 cups daily) is likely safe for most people.

Drinking more than 4 cups of black tea daily is possibly unsafe. Drinking large amounts might cause side effects due to the caffeine content. These side effects can range from mild to serious and include headache and irregular heartbeat.

Drinking very high amounts of black tea containing more than 10 grams of caffeine is likely unsafe. Doses of black tea this high might cause death or other severe side effects.

Pregnancy: Drinking moderate amounts of black tea during pregnancy is possibly safe. Do not drink more than 3 cups of black tea daily. This provides about 300 mg of caffeine. Consuming more than this during pregnancy is possibly unsafe and has been linked to an increased risk of miscarriage, increased risk of sudden infant death syndrome (SIDS), and other negative effects, including symptoms of caffeine withdrawal in newborns and lower birth weight.

Breast-feeding: Drinking moderate amounts of black tea while breast-feeding is possibly safe. But drinking more than 3 cups of black tea daily is possibly unsafe. The caffeine in black tea is passed into breastmilk. It might cause irritability and increased bowel movements in nursing infants.

Children: Black tea is possibly safe in children when consumed in amounts found in food and beverages.

Anxiety disorders: The caffeine in black tea might make anxiety worse.

Bleeding disorders: There is some concern that the caffeine in black tea might slow blood clotting. Use caffeine cautiously if you have a bleeding disorder.

Heart problems: Black tea contains caffeine. Large amounts of caffeine can cause irregular heartbeat in certain people. If you have a heart condition, use caffeine in moderation.

Diabetes: The caffeine in black tea might affect blood sugar. Use black tea with caution if you have diabetes.

Diarrhea: Black tea contains caffeine. The caffeine in black tea, especially when taken in large amounts, might worsen diarrhea.

Seizures: Black tea contains caffeine. High doses of caffeine might cause seizures or decrease the effects of drugs used to prevent seizures. If you have ever had a seizure, don’t take high doses of caffeine.

Glaucoma: Drinking caffeinated black tea increases the pressure inside the eye. The increase occurs within 30 minutes and lasts for at least 90 minutes. If you have glaucoma, use caffeine with caution.

Hormone-sensitive condition such as breast cancer, uterine cancer, ovarian cancer, endometriosis, or uterine fibroids: Black tea might act like estrogen. If you have any condition that might be made worse by exposure to estrogen, don’t use black tea.

High blood pressure: The caffeine in black tea might increase blood pressure in people with high blood pressure. But this doesn’t seem to occur in people who drink black tea or other caffeinated products regularly.

Irritable bowel syndrome (IBS): Black tea contains caffeine. The caffeine in black tea, especially when taken in large amounts, might worsen diarrhea and might worsen symptoms of IBS.

Brittle bones (osteoporosis): Drinking black tea can increase the amount of calcium that is flushed out in the urine. This might weaken bones. If you have osteoporosis, don’t drink more than 3 cups of black tea daily. If you are generally healthy and get enough calcium from your food or supplements, drinking about 4 cups of black tea daily doesn’t seem to increase the risk of getting osteoporosis.