There is no such thing as “side effects and contraindications of black tea”. It can be consumed in moderation by healthy people, but people with severe stomach problems, gastroesophageal reflux and insomnia need to reduce it. Black tea contains caffeine, theophylline, theobromine, flavonoids, theaflavin and theobromine which make it more flavorful, and of course some water-soluble vitamins and minerals. Some black teas are higher in caffeine. Caffeine can be refreshing, which is one of the reasons why people like black tea. But people who suffer from serious stomach problems as well as gastroesophageal reflux are prone to aggravate their reflux symptoms if they take too much black tea, and excessive black tea can also lead to insomnia. So in any case, everyone needs to drink black tea according to their own specific conditions, not too much.