Caffeine

Caffeine is a mild central nervous system stimulant and the most widely used psychoactive substance in the world. It occurs naturally in coffee, tea, cacao, and some other plants, and is added to soft drinks, energy drinks, and a range of over-the-counter medications, where it can help relieve drowsiness or boost the effect of some pain relievers. The U.S. Food and Drug Administration treats caffeine as both a drug and a food additive. For most healthy adults, the FDA points to about 400 milligrams a day — roughly two to three cups of coffee — as an amount not generally associated with harmful effects, though sensitivity varies and the figure is lower for pregnant people and not appropriate for children.

Caffeine works mainly by blocking adenosine, a brain chemical that builds up during the day and promotes drowsiness. By occupying adenosine receptors, caffeine prevents that 'slow down' signal, which leaves excitatory neurotransmitters more active and produces increased alertness and reduced fatigue. This also prompts the release of adrenaline, which contributes to caffeine's stimulant effects on the heart and circulation. With regular use the brain adapts, which is why tolerance develops and why stopping abruptly can produce withdrawal.

In typical amounts, caffeine increases alertness and reduces the feeling of fatigue, and can improve concentration. Higher doses can cause jitteriness, anxiety, restlessness, a faster or pounding heartbeat, upset stomach, and difficulty sleeping. Because caffeine has a real physical dependence profile, regular users who stop suddenly can experience withdrawal — most commonly headache, fatigue, low mood, irritability, and difficulty concentrating — usually within a day or so of stopping.

For most healthy adults, moderate caffeine intake is considered safe. The serious danger is dose. Symptoms of caffeine overdose include a rapid or erratic heartbeat, seizures, vomiting, diarrhea, disorientation, and in severe cases death. The greatest risk comes from pure or highly concentrated caffeine sold as bulk powder or liquid: the FDA has warned consumers to avoid these products and has linked them to deaths in otherwise healthy people, because a single teaspoon of pure powdered caffeine can equal the caffeine in roughly 28 cups of coffee, and the difference between an active and a lethal amount is too small to measure safely with kitchen tools. Caffeine can also be risky for people with certain heart conditions, for pregnant people, and when combined with alcohol or other stimulants. If a caffeine overdose is suspected, contact Poison Control at 1-800-222-1222, or call 911 in an emergency.