Pharmacology Of Caffiene

Caffeine is a central nervous system and metabolic stimulant,4  and is used both recreationally and medically to reduce physical fatigue and restore mental alertness when unusual weakness or drowsiness occurs. Caffeine stimulates the central nervous system first at the higher levels, resulting in increased alertness and wakefulness, faster and clearer flow of thought, increased focus, and better general body coordination, and later at the spinal cord level at higher doses.5  complex chemistry, and acts through several mechanisms as described below.

1. MECHANISM OF ACTION:

Caffeine's principal mode of action is as an antagonist of adenosine receptors in the brain.

Adenosine release mechanisms in the brain are complex.6 There is evidence that adenosine functions as a synaptically released neurotransmitter in some cases, but stress-related adenosine increases appear to be produced mainly by extracellular metabolism of ATP. It is not likely that adenosine is the primary neurotransmitter for any group of neurons, but rather that it is released together with other transmitters by a number of neuron types. McCarley and his colleagues have argued that accumulation of adenosine may be a primary cause of the sensation of sleepiness that follows prolonged mental activity, and that the effects may be mediated both by inhibition of wake-promoting neurons via A1 receptors, and activation of sleep-promoting neurons via indirect effects on A2A receptors.  More recent studies have provided additional evidence for the importance of A2A, but not A1, receptors.

Some of the secondary effects of caffeine are probably caused by actions unrelated to adenosine. Caffeine is known to be a competitive inhibitor of the enzyme cAMP-phosphodiesterase (cAMP-PDE), which converts cyclic AMP (cAMP) in cells to its noncyclic form, thus allowing cAMP to build up in cells. Cyclic AMP participates in activation of Protein Kinase A (PKA) to begin the phosphorylation of specific enzymes used in glucose synthesis.

2. METABOLISM & HALF LIFE
Caffeine is metabolized in the liver into three primary metabolites: paraxanthine (84%), theobromine (12%), and theophylline (4%).Caffeine from coffee or other beverages is absorbed by the stomach and small intestine within 45 minutes of ingestion and then distributed throughout all tissues of the body8. It is eliminated by first-order kinetics.9 Caffeine can also be ingested rectally, evidenced by the formulation of suppositories of ergotamine tartrate and caffeine (for the relief of migraine)10  and chlorobutanol and caffeine (for the treatment of hyperemesis).

The half-life of caffeine—the time required for the body to eliminate one-half of the total amount of caffeine—varies widely among individuals according to such factors as age, liver function, pregnancy, some concurrent medications, and the level of enzymes in the liver needed for caffeine metabolism. In healthy adults, caffeine's half-life is approximately 3–4 hours. In women taking oral contraceptives this is increased to 5–10 hours, 12  and in pregnant women the half-life is roughly 9–11 hours.13  Caffeine can accumulate in individuals with severe liver disease, increasing its half-life up to 96 hours.14  In infants and young children, the half-life may be longer than in adults; half-life in a newborn baby may be as long as 30 hours. Other factors such as smoking can shorten caffeine's half-life.15
Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system (specifically, the 1A2 isozyme) into three metabolic dimethylxanthines,  which each have their own effects on the body:

•    Paraxanthine (84%): Has the effect of increasing lipolysis, leading to elevated glycerol and free fatty acid levels in the blood plasma.
•    Theobromine (12%): Dilates blood vessels and increases urine volume. Theobromine is also the principal alkaloid in cocoa, and therefore chocolate.
•    Theophylline (4%): Relaxes smooth muscles of the bronchi, and is used to treat asthma. The therapeutic dose of theophylline, however, is many times greater than the levels attained from caffeine metabolism. Each of these metabolites is further metabolized and then excreted in the urine.

3. OVERUSE:
•    In large amounts, and especially over extended periods of time, caffeine can lead to a condition known as caffeinism.   
•    Caffeinism usually combines caffeine dependency with a wide range of unpleasant physical and mental conditions including nervousness, irritability, anxiety, tremulousness, muscle twitching (hyperreflexia), insomnia, headaches, respiratory alkalosis, and heart palpitations.              
•    Furthermore, because caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis, and gastroesophageal reflux disease.          
•    There are four caffeine-induced psychiatric disorders recognized by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition: caffeine intoxication, caffeine-induced anxiety disorder, caffeine-induced sleep disorder, and caffeine-related disorder not otherwise specified (NOS).

 3.1. Caffeine intoxication

An acute overdose of caffeine, usually in excess of about 300 milligrams, dependent on body weight and level of caffeine tolerance, can result in a state of central nervous system over-stimulation called caffeine intoxication,  colloquially "caffeine jitters". The symptoms of caffeine intoxication are not unlike overdoses of other stimulants. It may include restlessness, nervousness, excitement, insomnia, flushing of the face, increased urination, gastrointestinal disturbance, muscle twitching, a rambling flow of thought and speech, irritability, irregular or rapid heart beat, and psychomotor agitation.22 In cases of much larger overdoses mania, depression, lapses in judgment, disorientation, disinhibition, delusions, hallucinations and psychosis may occur, and rhabdomyolysis (breakdown of skeletal muscle tissue) can be provoked.

In cases of extreme overdose, death can result. The median lethal dose (LD50) given orally, is 192 milligrams per kilogram in rats. The LD50 of caffeine in humans is dependent on weight and individual sensitivity and estimated to be about 150 to 200 milligrams per kilogram of body mass, roughly 80 to 100 cups of coffee for an average adult taken within a limited time frame that is dependent on half-life. Though achieving lethal dose with caffeine would be exceptionally difficult with regular coffee, there have been reported deaths from overdosing on caffeine pills, with serious symptoms of overdose requiring hospitalization occurring from as little as 2 grams of caffeine. Death typically occurs due to ventricular fibrillation brought about by effects of caffeine on the cardiovascular system.

Treatment of severe caffeine intoxication is generally supportive, providing treatment of the immediate symptoms, but if the patient has very high serum levels of caffeine then peritoneal dialysis, hemodialysis, or hemofiltration may be required.

3.2 Anxiety and sleep disorders

Two infrequently diagnosed caffeine-induced disorders that are recognized by the American Psychiatric Association (APA) are caffeine-induced sleep disorder and caffeine-induced anxiety disorder, which can result from long-term excessive caffeine intake.
In the case of caffeine-induced sleep disorder, an individual regularly ingests high doses of caffeine sufficient to induce a significant disturbance in his or her sleep, sufficiently severe to warrant clinical attention.

In some individuals, the large amounts of caffeine can induce anxiety severe enough to necessitate clinical attention. This caffeine-induced anxiety disorder can take many forms, from generalized anxiety to panic attacks, obsessive-compulsive symptoms, or even phobic symptoms.

Because this condition can mimic organic mental disorders, such as panic disorder, generalized anxiety disorder, bipolar disorder, or even schizophrenia, a number of medical professionals believe caffeine-intoxicated people are routinely misdiagnosed and unnecessarily medicated when the treatment for caffeine-induced psychosis would simply be to stop further caffeine intake. A study in the British Journal of Addiction concluded that caffeinism, although infrequently diagnosed, may afflict as many as one person in ten of the population.