Caffeine's effects (pharmacological) on the body include central nervous system (CNS) stimulation, including a sense of well-being, decreased perception of fatigue, increased alertness, reduced boredom, a higher level of physical endurance, increased flow of thoughts and increased mental endurance (ability to continue working on intellectual activities). Caffeine affects the heart, increasing the pulse rate, and can have an effect on the cerebral (in the brain) blood vessels. If you ingest caffeine in doses higher than 100 mg, your production of gastric acids goes up and you may experience gastric irritation. (Since decaffeinated coffee also has this effect, caffeine is not the only ingredient which causes gastric disturbance.) The adverse effects of caffeine can include headaches, tremors, agitation, restlessness and insomnia. This is much more pronounced if you drink more than 10 cups of coffee a day.
Caffeine "dependence" is considered to be a weaker form of addiction, only because it features the three aspects of true addiction to a lesser degree. These aspects include having withdrawal symptoms, a tolerance to a specific dose of caffeine, needing to drink more and more coffee as time goes on and loss of control of the need to drink coffee. If you have stopped drinking coffee without tapering down your daily consumption, you could develop withdrawal symptoms which, while they may not be severe, may become intense. Caffeine creates its own continued use by relieving you of the withdrawal headache in the same way that alcohol cures the hangover headache (drinking the "hair of the dog that bit you"). Many headache remedies include caffeine in their lists of ingredients. Headache medication manufacturers include caffeine on purpose because caffeine improves the activity of the analgesics.
Caffeine-withdrawal headaches can last up to 48 hours and can be as intense as a migraine headache. Withdrawal symptoms can begin 12 to 24 hours after the last cup of coffee and can last up to 1 week. These symptoms can include lethargy, extreme nervousness, irritability, sweating, headaches and shakiness. Even with these symptoms, caffeine is considered to have a low potential for abuse as compared to other stimulants. The symptoms may disrupt your life for a short time if they are even moderately severe for you.