Caffeine dependence
Caffeine dependence is the condition of having a substance dependence on caffeine, a commonplace central nervous system stimulant drug which occurs in nature in coffee, tea, yerba mate, cocoa, and other plants. Caffeine is also an additive in many consumer products, including beverages such as caffeinated alcoholic beverages, energy drinks, and colas.
Caffeines mechanism of action is somewhat different from that of cocaine and the substituted amphetamines; caffeine blocks adenosine receptors A1 and A2A. Adenosine is a by-product of cellular activity, and stimulation of adenosine receptors produces feelings of tiredness and the need to sleep. Caffeines ability to block these receptors means the levels of the bodys natural stimulants, dopamine and norepinephrine, continue at higher levels.
The Diagnostic and Statistical Manual of Mental Disorders describes four caffeine-related disorders including intoxication, withdrawal, anxiety, and sleep.

Dependence
Mild physical dependence can result from long-term caffeine use.  Caffeine addiction, or a pathological and compulsive form of use, has been documented in humans.

Addiction vs. Dependence
Caffeine use is classified as a dependence, not an addiction. For a drug to be considered addictive, it must activate the brains reward circuit. Caffeine, like addictive drugs, enhances dopamine signaling in the brain (is eugeroic), but not enough to activate the brains reward circuit like addictive substances such as cocaine, morphine, and nicotine. Caffeine dependence forms due to caffeine antagonizing the adenosine A2A receptor, effectively blocking adenosine from the adenosine receptor site. This delays the onset of drowsiness and releases dopamine.Studies have demonstrated that people who take in a minimum of 100 mg of caffeine per day (about the amount in one cup of coffee) can acquire a physical dependence that would trigger withdrawal symptoms that include headaches, muscle pain and stiffness, lethargy, nausea, vomiting, depressed mood, and marked irritability.  Professor Roland R. Griffiths, a professor of neurology at Johns Hopkins in Baltimore strongly believes that caffeine withdrawal should be classified as a psychological disorder. His research suggested that withdrawal affects 50% of habitual coffee drinkers,  beginning within  12–24 hours after cessation of caffeine intake, and peaking in 20–48 hours, lasting as long as 9 days.Continued exposure to caffeine leads the body to create more adenosine-receptors in the central nervous system, which makes it more sensitive to the effects of adenosine. It reduces the stimulatory effects of caffeine by increasing tolerance, and it increases the withdrawal symptoms of caffeine as the body becomes more sensitive to the effects of adenosine once caffeine intake decreases. Caffeine tolerance develops very quickly. Tolerance to the sleep disruption effects of caffeine were seen after consumption of 400 mg of caffeine 3 times a day for 7 days, whereas complete tolerance was observed after consumption of 300 mg taken 3 times a day for 18 days.

Physiological effects
When a person becomes caffeine dependent, it can cause a person to suffer different physiological effects which could lead to withdrawal. Symptoms can range from mild to severe that are dependent on the amount of caffeine consumed daily. Tests are still being done to get a better understanding of the effects that occur to someone when they become dependent on different forms of caffeine to make it through the day.

Adults
Caffeine is consumed by many each day and from this a dependence is formed. For the most part caffeine consumption is safe, but consuming over 400 mg of caffeine has shown adverse physiological and psychological effects; especially in people that have pre-existing conditions. When adults form a dependence on caffeine, it can cause a range of health problems such as headaches, insomnia, dizziness, cardiac issues, hypertension and others. When an adult is dependent on this substance, they must consume a certain amount of caffeine every day to avoid these effects.

Pregnancy
If pregnant, it is recommended not to consume over 200 mg of caffeine a day (depending on size of person). If a pregnant female consumes high levels of caffeine, it can result in low birth weights due to loss of blood flow to the placenta which could lead to an increase health problems later in that childs life. It can also result in premature labor, reduced fertility, and other reproductive issues. If a female is depended on an excessive amount of caffeine to get them through the day, it is recommended to talk to their healthcare provider to either eliminate the dependency of caffeine or to become less dependent on it.

Children and Teenagers
It is not recommended for children under the age of 18 to consume several caffeinated drinks according to the American Academy of Pediatrics (AAP). If they were to consume caffeine it is recommended to follow some guidelines so they do not consume too much throughout the day. If they do, they can become dependent on caffeine and without it can suffer many different side effects. These include increase of heart rate and blood pressure, sleep disturbance, mood swings, and acidic problems. Long lasting problems on childrens nervous system and cardiovascular system are currently unknown and studies are still being conducted on it.

Tolerance
Tolerance levels regarding caffeine typically vary from person to person. Caffeine tolerance occurs when the stimulatory effects of caffeine decrease over time due to regular consumption. According to H.P. Ammon from the National Library of Medicine, caffeine tolerance occurs when the body responds to an intake of caffeine through the up-regulation of adenosine-receptors.

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