Black tea can be drunk in moderation, but not in large quantities for a long time. Because black tea contains a large amount of tannic acid, tannic acid can react with the iron in food, generating insoluble substances, which affects the absorption of iron in the body, causing iron deficiency in the body and inducing iron deficiency anemia. Secondly, black tea has an obvious effect on lowering the three highs, and if you are a patient with low blood sugar and low blood pressure, you should drink less black tea, otherwise it will cause dizziness, nausea and other problems. Furthermore, black tea has a high fluorine content, too much of it will cause calcium ions in the body not to be easily absorbed, which can easily cause calcium deficiency, leading to osteoporosis and other diseases. Black tea also has a refreshing effect, can relieve fatigue, but insomnia people better not drink, otherwise it will make insomnia symptoms aggravated.