Many people don\u2019t consider caffeine a drug, but it is the most commonly used mood-altering drug in the world. While about 1.6 billion cups of coffee are consumed worldwide a day, tea, chocolate, cocoa beverages, soft drinks and energy drinks also contain varying amounts of caffeine. The stimulant effects of caffeine on the central nervous system have been known for centuries. In the 15th century, the stimulatory effects of drinking coffee gave rise to an abundance of coffee houses all over Arabia. In the 19th century, the famous French writer Honor\u00e9 De Balzac was addicted to coffee, drinking 50 cups a day by the time he wrote his most famous work The Human Comedy. In an 1830 article titled \u201cPleasures and pains of coffee,\u201d he said, \u201cCoffee slips into the stomach and you immediately feel a general commotion. Ideas begin to move like the battalions of the Grand Army on the field of the battle and the battle takes place. Memories come at a gallop, carried by the wind.\u201d The Effects of Caffeine After ingestion, caffeine is quickly absorbed through the small intestine into the circulatory system and blood. Maximum blood concentration occurs about 30-60 minutes after consumption. Since it is both water- and fat-soluble, caffeine promptly infiltrates all body tissues and crosses the blood-brain, blood-placenta and blood-testis barriers. On average, the half-life of caffeine is 4-6 hours, however due to varying factors such as age, liver health, pregnancy status, individual absorption and metabolism, the half-life range of caffeine is 2-12 hours. Caffeine can impart its stimulatory effects as quickly as 15 minutes after consumption and last as long as 6 hours. Individual sensitivity to caffeine also varies. For example, people with a specific variation of the gene PDSS2 process caffeine more slowly than others; therefore, they require less to attain the same stimulant effects. In moderate doses (32-300 mg), caffeine helps increase attentiveness, reaction time and alertness, while reducing sleepiness. Studies on severely fatigued people found caffeine improved mood and reduced anxiety. Regular, excessive caffeine consumption can lead to poor concentration, nervousness, heartburn, constipation and diarrhea. Long-term effects may include sleep deprivation, impaired judgment, emotional fatigue, mood swings, depression and anxiety. Caffeine Intake During Pregnancy and Breastfeeding Research is inconclusive, however, some studies suggest more than 200 mg of caffeine a day increases the risk of miscarriage and low birth weight, while drinking eight cups of coffee or more a day has been linked to stillbirth. The American College of Obstetricians and Gynecologists recommends pregnant women limit caffeine consumption to fewer than 200 mg per day. When caffeine enters the bloodstream, usually less than 1% ends up in breast milk, with the amount in milk peaking a couple of hours after consumption. Newborn infants cannot easily break down and eliminate caffeine, so it may accumulate in their systems. Many experts state no more than 300 mg per day is acceptable for nursing moms. The Impact of Caffeine on the Brain Structurally, caffeine closely resembles adenosine, a molecule occurring naturally in the brain. The resemblance is so close, caffeine can fit neatly into the brain cells\u2019 receptors for adenosine. When adenosine binds to enough receptors, it signals the brain it is time to rest or sleep. Caffeine masks drowsiness symptoms because it blocks adenosine production. When adenosine is blocked by caffeine, this causes a surge in the feel-good neurotransmitter dopamine. Moreover, increased levels of adenosine in the blood trigger the stimulating hormone adrenaline, boosting alertness and energy. This explains why people who drink caffeinated beverages too close to bedtime might have trouble falling asleep. Caffeine Addiction and Withdrawal Caffeine causes brain chemistry changes in people who consume it on a daily basis. Like drugs, people become dependent and tolerance builds over time, so more caffeine is required to produce its energizing effects. The ever-increasing varieties of high-caffeine energy drinks is linked to a rise in caffeine abuse and dependence, especially among teens and young adults. Misuse can lead to caffeine intoxication, putting individuals at risk for premature and unnatural death. These dangers are compounded when energy drinks are mixed with alcohol. Although caffeine dependence is not associated with the same perils as drug addiction, quitting caffeine use, especially when abrupt, can lead to withdrawal symptoms. People who stop consuming caffeine completely can experience any of the following withdrawal symptoms for 7-12 days: \tHeadache \tSleepiness \tIrritability \tLethargy \tConstipation \tDepression \tMuscle pain, stiffness or cramping \tLack of concentration \tFlu-like symptoms \tInsomnia \tNausea and vomiting \tAnxiety \tBrain fog \tDizziness Caffeine does not have the negative stigma of illicit drugs, however, it can be addictive and excessive use can lead to detrimental physical and psychological side effects. Gradual tapering is advised to reduce the severity of withdrawal symptoms.