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  • General increase in attention: Research has failed to show improvement in substantive memory or complex cognitive tasks [Smith 2002], although research has shown caffeine slightly improves retention (Borota et al., 2014) on simple memory tasks.
  • Increases anxiety and nervousness at high doses or in sensitive individuals [Lieberman 1992, Loke 1988]
  • Insomnia, decreased ability to sleep [Smith 2002]
  • Caffeine withdrawal can worsen mood [Schuh 1997]
  • Caffeine withdrawal causes headaches, flu-like symptoms, feelings of lethargy and reduces motivation [Schuh 1997]
  • Increases jaw tension and bruxism [Erowid Self Reports 2000, Ohayon 2001]
  • Although caffeine can increase seizure threshold (decrease likelihood of seizures) in some people, other epileptics say that caffeine can trigger seizures.

What are the short term effects of caffeine on the body?

Caffeine increases heartbeat, respiration, basal metabolic rate, gastroenteric reflexes, and the production of stomach acid and urine; and it relaxes smooth muscles, notably the bronchial muscle. All of these changes vary considerably among people and may depend upon the individual's sensitivity to this drug, his/her metabolism, or upon whether the consumer habitually uses or rarely uses caffeine. How long caffeine's effects last is influenced by the person's hormonal status, whether he/she smokes or takes medications, or has a disease that impairs liver functioning.

Subjectively, people report that caffeine gives them a "lift." They feel less drowsy, less fatigued, more capable of rapid and sustained intellectual effort. They also report improved performance of some manual tasks such as driving. However, caffeine may restore only those abilities or feelings the person had before fatigue or boredom set in. Studies have also shown that caffeine decreases reaction time to both visual and auditory stimuli; it does not significantly alter numerical reasoning (arithmetic skills) or short-term memory; and it can diminish performance of manual tasks that involve delicate muscular coordination and accurate timing.

When caffeine is taken in high doses it can cause many unwanted side effects. To learn more about these, please read "What are the symptoms of caffeine overdose?"

How long does it take to feel caffeine's effects?

Caffeine is rapidly absorbed through the stomach and small intestine into the bloodstream, where it takes about 15-45 minutes to reach peak levels. The level of caffeine in the blood reaching the brain determines the potency of its effects on the body. Usually the central nervous system is maximally stimulated within 30-60 minutes. Caffeine is metabolized by the liver and eventually its metabolites are excreted in urine, although caffeine can also be secreted in saliva, semen, and breast milk.

Caffeine will continue to have an effect on the body as long as it remains in the blood. The time required for the body to eliminate one-half of the total amount of caffeine consumed (or caffeine's half-life) varies from several hours to several days, but for the average non-smoking adult it is about 3-4 hours.1, 2 Several factors can lengthen caffeine's half-life, such as some medications, liver diseases, pregnancy, and the level of enzymes in the liver needed for caffeine metabolism. The half-life of caffeine in a pregnant women is 18-20 hours; the half-life in women taking oral contraceptives is up to 13 hours. Other factors, such as smoking can shorten caffeine's half-life. Half-life in a newborn baby may be as long as 30 hours.2

What are the symptoms of caffeine overdose?

Caffeine stimulates the central nervous system and can produce a variety of effects elsewhere in the body. The symptoms of a caffeine overdose ("caffeinism") will vary, according to individual differences and the amount consumed. Doses ranging from 250 to 750 mg (2 to 7 cups of coffee or 1-4 tablets of NoDoz) can produce restlessness, dizziness, nausea, headache, tense muscles, sleep disturbances, and irregular heart beats. Doses of over 800 mg (7 cups of coffee or 4 tablets of NoDoz) can produce all of the above as well as a reaction similar to an anxiety attack, including delirium, drowsiness, ringing ears, diarrhea, vomiting, light flashes, difficulty breathing, convulsions (extreme overdose). These amounts of caffeine may come from a single dose or from multiple doses at short intervals. Besides caffeine's effects, the essential oils of coffee may cause gastrointestinal irritation and diarrhea, and the high tannin content of tea can result in constipation.

How does caffeine affect sleep?

Studies of the effect of caffeine on sleep have shown that one strong cup of coffee, drunk 30-60 minutes before going to sleep, can cause restlessness and difficulty falling asleep, increased body movements, a tendency to be awakened more readily by sudden noises, and a decreased quality of sleep.

The wide variation in caffeine's effects on sleep may be related to whether the person is a habitual, heavy user or a non-user of caffeine. Non-users report greater difficulty with sleep after consuming caffeine. Heavy users may also experience sleep disturbances; however, their increased tolerance to the effects of caffeine affects their perceptions of disturbed sleep. Often they sleep for shorter periods of time, but have grown accustomed to this pattern.

If you use caffeine and are having trouble sleeping, you may want to try cutting back on your total daily intake or avoid consumption 5-7 hours before you plan on sleeping, to allow time for the caffeine to be metabolized.

Caffeine and Health Problems

HEARTBURN - In some people, coffee has been shown to relax the sphincter muscles at the lower end of the esophagus, allowing the stomach contents to "back up" into the esophagus; this is called heartburn. Caffeine alone has no effect on the sphincter muscles, so other substances in coffee apparently cause the heartburn.

ULCERS - Caffeine can increase the secretion of both acid and pepsin in the stomach, which could exacerbate an existing ulcer. However, since both "regular" and decaffeinated coffees have also been shown to stimulate the gastric mucosa and increase stomach acid secretion, caffeine is probably not the only component of coffee responsible.

FIBROCYSTIC BREAST DISEASE - In the late '70's and '80's, several well-publicized studies linked caffeine consumption with benign fibrocystic breast disease (FBD). More recent investigations, however, have not supported these findings. Individual women may notice a decrease in palpable breast lumps, the symptoms of FBD, when they limit their caffeine intake, although research has not shown a direct association.

HEART PROBLEMS - Caffeine has been shown to cause cardiac arrhythmias (irregular heartbeats) in certain individuals; often persons with heart disease are told to avoid caffeine altogether. However, no evidence shows that caffeine is a causative factor in the development of heart disease. Also see "Has coffee drinking been linked to heart disease?" in this section.

HEART DISEASE - The relationship between coffee drinking and heart disease has been investigated in numerous studies but strong, independent risks associated with coffee drinking have not been clearly documented. Researchers at Johns Hopkins Medical Institute recently found heavy coffee drinkers (defined as five or more cups per day) were two to three times more likely to have coronary heart disease than were nondrinkers (NEJM, October 16, 1986). This relationship was true even when accounting for other important risk factors such as age, smoking habits, serum cholesterol and blood pressure.

However, the study did not control for the effects of diet, sedentary lifestyle or high levels of occupational stress. Nor did it indicate the exact quantity in ounces of coffee consumed, the type of coffee (regular or decaffeinated), the brewing method, or the use of cream or sugar.

If coffee drinking does increase the risk of heart disease, it may do so through its effect on cholesterol. A few studies have linked heavy coffee consumption to elevated total serum cholesterol, although caffeine alone does not seem to be responsible. Perhaps it's not even the coffee, but a high saturated fat diet among heavy coffee drinkers that's increasing the risk of heart disease. Increased coffee consumption by men has been associated with increased calorie consumption from fat and increased cholesterol intake. (Adapted from DNS ALERT, Cornell Cooperative Extension, Dec. 1986)

Addiction and Withdrawal

Caffeine is addictive according to several definitions of 'addiction'. It causes withdrawal symptoms after cessation of heavy use (most commonly headaches) and regular users develop tolerance and experience cravings when ceasing use. Regular users can also become emotionally and mentally dependent upon their daily caffeine (coffee, soda, etc).

Withdrawal Symptoms
Caffeine has the potential to produce tolerance, which means that increased amounts of the drug are needed to achieve a consistent effect. Withdrawal symptoms can occur when use of caffeine is stopped abruptly. Users may experience fatigue, and most commonly, headaches. Primary withdrawal effects last for only a few days though mild withdrawal effects can last as long as a week or two.

  • craving
  • tiredness, lethargy
  • confusion and inability to focus
  • headaches, which may not respond to painkillers, but may abate with the intake of caffeine
Erowid's effects information is a summary of data gathered from users, research, and other resources. This information is intended to describe the range of effects people report experiencing. Effects may vary dramatically from one person to another or one experience to another based on a variety of factors such as body chemistry, age, gender, physical health, dose, form of material, etc.