Caffeine is a bitter plant alkaloid that acts as a mild central nervous system stimulant and as a diuretic. It is found especially in coffee beans, tea leaves, cacao beans, and kola nuts. Caffeine also is available in capsules and tablets and is added to soft drinks and energy bars and drinks. It is the most widely used psychoactive substance in the world.
Caffeine is used to increase mental alertness and temporarily relieve fatigue. Throughout the world many people start their days with caffeine. It has been shown to improve short-term memory, enhance concentration, speed-up reaction time, and increase one’s capacity for physical labour.
These effects are all temporary. Caffeine does not replace the need for rest or sleep; nor does it boost functioning above normal levels. In habitual caffeine users its effects are due, in part, to the prevention of caffeine withdrawal.
In addition to being an ingredient in various foods and beverages such as coffee, tea, soft drinks, and chocolate, caffeine is used medicinally, both alone and as an additive in other drugs. It is added to some antihistamines to counteract drowsiness—a common antihistamine side effect. It is added to various over-the- counter (OTC) and prescription painkillers—especially headache remedies—to enhance their effectiveness. Some weight-loss medications and supplements also contain caffeine. Prescription citrated caffeine is used to treat breathing problems in premature infants.
About 60 different plants produce caffeine as a natural pesticide against insect predators. The compound was first purified from coffee by the German chemist Friedrich Ferdinand Runge in 1819. In addition to coffee, tea, cacao, and kola nuts, caffeine is derived from yerba mate (Ilex paraguariensis) and guarana (Paullina cupana) berries.
Humans have consumed caffeine for thousands of years—chewing seeds or leaves of caffeine-producing plants or cooking them to prepare caffeine-containing beverages. Coffee was introduced to Europe from the Middle East in the seventeenth century and soon rivalled alcohol as the social beverage of choice. It is estimated that 90% of North Americans now consume caffeine on a daily basis.
Although caffeine has no nutritional value, the U.S. Food and Drug Administration (FDA) categorizes it as ‘‘generally recognized as safe’’ (GRAS) and recent research suggests that moderate amounts of caffeine may have some health benefits. It is absorbed from the stomach into the bloodstream where it:
- increases heart rate
- temporarily increases blood pressure
- relaxes smooth muscle cells in the airways
- releases fatty acids and glycerol for energy
- easily crosses the blood-brain barrier and affects the levels of neurotransmitters in the brain
- increases urine output
Individuals vary greatly in their sensitivity to caffeine and in the length of time that it remains in the body. Caffeine’s effects are usually noticeable about 15 minutes after ingestion and typically last several hours. On average, one-half of ingested caffeine is eliminated from the body within three to four hours.
Moderate daily caffeine consumption—300–400 milligrams (mg), about 3–4 cups of coffee—is generally considered to be safe. However it can be difficult to determine an individual’s caffeine consumption:
- A large number of products contain caffeine.
- Although caffeine must be listed as an ingredient on U.S. food labels, disclosure of the amount of caffeine per serving is not required.
- The caffeine content of coffees and teas varies greatly depending on the plant source, the location where the plants are grown, and how the beverages are prepared.
Approximate amounts of caffeine is some common products include:
- brewed coffee, 8 ounces (oz) or 240 milliliters (mL): 95–200 mg
- espresso coffee, 1 oz (30 mL): 58–75 mg
- brewed decaffeinated coffee, 8 oz (240 mL): 2–12 mg
- brewed black tea, 8 oz (240 mL): 40–120 mg
- brewed green tea, 8 oz (240 mL): about 15 mg
- decaffeinated tea, 8 oz (240 mL): 1–4 mg
- Mountain Dew, 12 oz (355 mL): 54 mg
- Coca-Cola Classic, 12 oz (355 mL): 35 mg
- Diet Coke, 12 oz (355 mL): 47 mg
- Pepsi, 12 oz (355 mL): 36–38 mg
- Sunkist Orange, regular or diet, 12 oz (355 mL): 41 mg
- Barq’s Root Beer, 12 oz (355 mL): 23 mg
- Dr Pepper, 12 oz (355 mL): 42–44 mg
- Sprite, Fanta, 7Up, 12 oz (355 mL): 0 mg
- No Name (formerly Cocaine) energy drink, 8.4 oz (250 mL): 280 mg
- Red Bull energy drink, 8.3 oz (245 mL): 76 mg
- SoBe No Fear energy drink, 8 oz (240 mL): 83 mg
- Hershey’s Special Dark Chocolate, 1.45 oz (41 g): 31 mg
- Hershey’s Milk Chocolate, 1.55 oz (43 g): 9 mg
- Excedrin extra-strength headache tablet: 65 mg
- NoDoz maximum-strength caffeine tablet: 200 mg
Although caffeine is poisonous to dogs, horses, and some birds, in moderate amounts it is not usually harmful to humans and may even have health benefits in adults. However caffeine sensitivity is affected by weight, age, and various medications and there are large individual differences in reactions to caffeine. It is possible to overdose on caffeine and an overdose of caffeine pills is potentially fatal.
The mental and physical benefits of caffeine are temporary and can be followed by a ‘‘crash’’ when the caffeine wears off. Those who use caffeine to stay awake while driving or operating heavy machinery are at risk for accidents from fatigue once the effects dissipate. Many people quickly develop tolerance to the effects of caffeine, along with mild physical and psychological dependencies. Discontinuing caffeine after regular use can cause withdrawal symptoms, especially headaches, within 12–24 hours. Other withdrawal symptoms can include irritability, nausea, inability to concentrate, sleepiness, fatigue, and mild depression. Withdrawal symptoms peak at about 48 hours and can last up to five days. Tapering off on caffeine—such as reducing consumption by one-half cup of coffee (about 50 mg) per day—minimizes or eliminates withdrawal symptoms.
The American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM) recognizes four different syndromes that can result from heavy overuse of caffeine:
- Caffeine intoxication is usually the result of taking caffeine pills (e.g., NoDoz). It is characterized by mental changes, rambling thoughts and speech, irregular heart beat, and all symptoms associated with caffeine overuse. In severe cases death can result from ventricular fibrillation (unsynchronized contractions of the heart ventricle).
- Caffeine-induced anxiety disorder is severe anxiety that interferes with daily social interactions and occurs after caffeine intoxication or heavy long-term use of caffeine.
- Caffeine-induced sleep disorder is insomnia that requires medical/psychiatric attention and occurs after prolonged caffeine consumption.
- Non-specific caffeine-induced disorder is an otherwise unspecified disorder that is associated with either acute or long-term caffeine consumption.
Children and teens are particularly sensitive to caffeine. Most children obtain their caffeine from soft drinks. Therefore beverages such as water, fruit juice, low-fat milk, or—at the least—caffeine-free sodas should be substituted for caffeine-containing soft drinks. Accidental ingestion of caffeine pills by children is a medical emergency. Coffee drinking often begins during adolescence and many teenagers consume caffeine- containing energy drinks and energy bars as well as coffee, leading to adverse effects.
The elderly also may be particularly sensitive to the adverse effects of caffeine.
Pregnant or breastfeeding
Caffeine has not been shown to cause birth defects and moderate amounts are considered safe during pregnancy. However caffeine may be eliminated from the body at a much slower rate in pregnant women. It is generally recommended that women limit their caffeine intake to the equivalent of two cups of coffee daily during pregnancy. Women who are having difficulty becoming pregnant should consider eliminating caffeine. Caffeine passes into breast milk and can cause restlessness, irritability, and sleeplessness in infants.
Other conditions and allergies
Patients with high blood pressure may be more susceptible to adverse effects of caffeine. Liver damage slows the elimination of caffeine from the body. Patients with these conditions should carefully monitor their caffeine intake.
Caffeinism is a group of symptoms caused by excess consumption of caffeine. Although the amount of caffeine required for these side effects varies with the individual, caffeinism generally develops in people who consume more than about 500 mg daily. Symptoms can be similar to those of caffeine withdrawal and may include:
- muscle twitching
- racing heart
- increase the effectiveness of OTC and prescription medicines for migraines and other headaches
- increase urine output in people taking diuretics (water pills)
- take up to six hours to be eliminated from the bodies of women taking oral contraceptives
- be broken down more slowly in patients taking the antibiotics ciprofloxacin (Cipro) and norfloxacin (Noroxin)
- increase the concentration of theophylline in the blood
- increase the risks of ephedra (ma-huang) in herbal teas or banned dietary supplements