Caffeine is a central nervous system stimulant that can temporarily reduce drowsiness and increase alertness. Caffeine-containing products such as coffee, tea, soft drinks, chocolate, and energy drinks are extremely popular, making caffeine one of the most widely consumed psychoactive substances in the world.

Coffee and tea are two of the most common natural sources of caffeine, although the amount varies considerably depending on the type of product and how it is prepared. Caffeine is also found naturally in ingredients such as cocoa, guarana, and yerba mate. Energy drinks and other foods may also contain added caffeine.

Unlike some other central nervous system stimulants, the effects of caffeine are temporary. For most people, moderate caffeine consumption can increase alertness and attention. However, sensitivity varies from person to person, and consuming caffeine later in the day can interfere with sleep.

Regular caffeine consumption can cause the body to become accustomed to caffeine. If someone who regularly consumes caffeine suddenly stops, withdrawal symptoms such as headache, fatigue, irritability, and difficulty concentrating may occur. These symptoms are generally temporary, and gradually reducing caffeine intake can make withdrawal easier.

Research has found an association between caffeine consumption and a lower risk of developing Parkinson’s disease in some populations. However, this does not establish that caffeine prevents Parkinson’s disease, and more research is needed to understand the relationship.

Excessive caffeine intake can cause symptoms such as nervousness, irritability, anxiety, insomnia, headache, increased heart rate, heart palpitations, and stomach upset. Health Canada recommends a maximum daily caffeine intake of 400 milligrams for adults 18 and older. The recommended maximum is lower for children and adolescents, and Health Canada recommends no more than 300 milligrams per day for people who are pregnant or breastfeeding. Individual sensitivity can vary, so some people may experience unpleasant effects at much lower amounts.

Very large doses of caffeine, particularly from concentrated caffeine products, can cause serious toxicity, including seizures and dangerous heart-rate or rhythm changes. Pure or highly concentrated caffeine products can be especially hazardous because a relatively small measuring error can result in a very large dose.

作者：Michael Chiu，MWS，CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026