Consumed in moderation, black tea is beneficial. Besides temporarily boosting alertness, black tea contains helpful substances that may prevent a number of diseases and conditions, including reducing stress and lowering blood pressure.
Black tea contains caffeine and theophylline, which are stimulants that help tea drinkers to feel more awake and energetic. It is also high in polyphenols, antioxidants known to prevent the DNA damage that leads to cancer.
According to WebMD, some studies have shown that women who drink black tea have lower rates of ovarian cancer than women who don't, and other studies have shown that black tea reduces the levels of stress hormones.
Black tea is thought to help reduce atherosclerosis, high cholesterol and to slow or prevent osteoporosis. It may also help prevent the formation of kidney stones. Some studies suggests there is a link between black tea and a reduced risk of lung cancer, Parkinson's disease and diabetes, according to WebMD.
Boston University found that consuming black tea reverses certain aspects of coronary-artery disease that are precursors to heart attacks. This benefit from black tea only occurs if dairy products are not added.
However, drinking too much black tea can cause insomnia, anxiety, headaches, nausea, heart arrhythmia or muscle tremors. Additionally, combining it with other sources of caffeine or other stimulants, such as ephedra, is considered unsafe. Excess stimulants can cause seizures and may lead to loss of consciousness.