One of the causes of headache is caffeine withdrawal. Say you failed to take your coffee and now suffering from headache, then your caffeine withdrawal is responsible for that.
Caffeine is a widely used stimulant, it is readily available and cheap. Causes of withdrawal symptoms include headache, weakness, muscle pain, difficulty concentrating, or a feeling like with a fever.
Recently, an article was published saying that researchers are now classifying caffeine withdrawal as a psychiatric disorder.