Caffeine is a stimulant. It can cause increased heart rate, increased blood pressure, irregular heart beats, nervousness, anxiety, insomnia and an increase in stomach acid. Caffeine is also addictive and withdrawal can cause headaches, lethargy and depression. The average cup of coffee contains 80 to 150 milligrams of caffeine. 40 milligrams per kilogram of body weight is toxic for a child.
High doses can cause delirium, disorientation and seizures.
Another problem with coffee is that it contains acids that are hard on the stomach. The acids may cause stomach pain and acid reflux or heartburn. This is especially true of the stronger coffees. This term is used to refer to a Turkish type of coffee which is quite strong. It may not have any more caffeine than regular coffee but does have more of the harmful acids.