Why might a person who suddenly stops drinking coffee get headaches for a couple of days?

Caffeine is a bitter substance found naturally in more than 60 plants, including coffee beans, tea leaves, kola nuts and the cacao pods used to make chocolate. It is also made in factories and added to some pain relievers, cold medicines, energy drinks and snacks.

The amount of caffeine in a drink varies a great deal. A cup of coffee of about 240 millilitres has 95 to 200 milligrams, the same amount of tea has 14 to 60 milligrams, and an energy drink of that size has 70 to 100 milligrams. A can of cola, about 355 millilitres, has 35 to 45 milligrams.

Caffeine acts on the brain and nerves, making people feel more awake and full of energy. It also has other effects. It makes the body pass more urine, getting rid of extra salt and water; it increases the acid in the stomach, which can cause an upset stomach; and it raises blood pressure.

For most adults, up to 400 milligrams a day is generally not harmful. Too much can cause restlessness, shakiness, sleeplessness, headaches, dizziness, a fast heartbeat, dehydration and anxiety. People who suddenly stop taking caffeine may have headaches, feel drowsy and irritable, and find it hard to concentrate for a couple of days. Children and teenagers are more sensitive to caffeine and should have less than adults.