Caffeine — natural stimulant in coffee, tea and other plants

Characteristics and how it works

Caffeine is an alkaloid that is rapidly absorbed after oral consumption. Its principal mode of action is antagonism of adenosine receptors in the brain, which reduces drowsiness and can enhance concentration. Effects begin within minutes for many people and may last several hours; individual factors such as age, body weight and liver function alter how long it remains active. Regular use produces tolerance and can lead to mild physical dependence and withdrawal symptoms such as headaches and irritability.

Natural sources and common products

Caffeine occurs naturally in a variety of plants, where it likely evolved as a pest deterrent. Human consumption typically comes from beverages and some foods. Common sources include:

• Coffee — the most familiar source worldwide, with varying caffeine content by bean and preparation.
• Tea — many varieties contain caffeine alongside other active compounds.
• Soft drinks — many cola-style sodas include added caffeine.
• Energy drinks — often formulated with higher caffeine and other stimulants.

History, cultural role and uses

Humans have consumed caffeine-containing plants for centuries in social, religious and medicinal contexts. Coffee houses and tea ceremonies influenced social life in many cultures. Today caffeine is used both recreationally and functionally — to improve wakefulness, support short-term mental performance, and as an ingredient in some over-the-counter medicines for headaches or cold symptoms.

Effects, safety and risks

At moderate doses most people experience increased alertness, improved reaction time and temporary mood elevation. Higher doses can cause restlessness, rapid heartbeat, insomnia and gastrointestinal discomfort. Abrupt cessation of regular use commonly produces withdrawal symptoms. Extremely large single doses can be dangerous; estimates that consuming the equivalent of tens of grams — often cited as roughly the caffeine in 80–100 cups of coffee — could produce a life-threatening overdose are widely reported and illustrate the potential hazard of concentrated products or accidental ingestion.

As a social and pharmacological phenomenon, caffeine remains the world's most widely used psychoactive substance, legal in nearly every country. Variability in metabolism and sensitivity means recommended intake differs among individuals; moderation and awareness of sources are the usual precautions.