Instant brewed iced tea typically contains 11 to 47 milligrams of caffeine per 8-ounce serving, while bottled iced tea beverages can contain 5 to 40 milligrams of caffeine per 8-ounce serving. Excessive intake of caffeine can cause headaches, restlessness and anxiety.
Total caffeine intake in adults should not exceed 500 milligrams per day. For adolescents, the American Academy of Pediatrics recommends that caffeine intake should not exceed 100 milligrams per day. Black tea contains the most caffeine of all teas, with 14 to 70 milligrams per 8-ounce serving, while green tea contains 24 to 45 milligrams of caffeine per 8-ounce serving.