Coffee contains a complex mixture of chemical compounds. Some components, particularly those related to the aroma, are produced during roasting of the green beans. The substances which during "brewing" dissolve in water to form the beverage are classified as nonvolatile taste components (including caffeine, trigonelline, chlorogenic acid, phenolic acids, amino acids, carbohydrates, and minerals) and volatile aroma components including organic acids, aldehydes, ketones, esters, amines, and mercaptans.

The major physiologically active substance in coffee is the alkaloid caffeine (C_8 H_10 O_2 N_4·H_2 O), also called guaranine or methyltheobromine, which acts as a mild stimulant. Caffeine is a naturally occurring substance found in the leaves, seeds or fruits of more than 60 plants, including coffee and cocoa beans, cola nuts and tea leaves. These are used to make beverages such as coffee, tea and cola drinks, and foods such as chocolate. Caffeine is also contained in many over-the-counter (OTC) and prescription medications. In the United States, most of the population uses caffeine in some form.

A cup of coffee, depending on the strength, may contain some 20-100mg of caffeine. Some types of coffee may also contain significant amounts of the B-vitamin niacin, although this nutrient is of course readily available from other foods as well. Caffeine-containing tablets or medications should not be taken as well as cups of coffee or tea, since this would increase the true dosage. The effects of caffeine vary from person to person; some individuals can drink several cups of coffee in an hour and notice no effects, while others may feel a strong effect after just one serving. Caffeine is prohibited for competition athletes.
People who wish to avoid or minimise caffeine intake (see below) often use decaffeinated coffee, or coffee substitutes. One method of decaffeination is by treating the green beans (before roasting) with chlorinated hydrocarbon solvents; other methods are also used. Important coffee substitutes are chicory, and roasted cereals such as barley; although these are commonly used not as total substitutes but as "extenders". Under U.S. law, the addition of chicory or any other substance must be clearly stated on the label.

Short-term effects

Caffeine is a drug that has been widely used for centuries. Its main
effect is that it is a mild stimulant of the central nervous system (CNS), helping to reduce feelings of drowsiness and fatigue. However, regular use may lead to "habituation"; that is, no net benefit from use but, rather, a negative effect if the drug is not taken.

Besides the above-mentioned CNS stimulant effect, caffeine also temporarily increases heartbeat, increases the blood pressure, and stimulates the action of the lungs; increases basal metabolic rate (BMR), and promotes urine production; and it relaxes smooth muscles, notably the bronchial muscle. Caffeine is used in treating migraine, either alone or in combination. It enhances the action of the ergot alkaloids used for the treatment of this problem, and also increases the potency of analgesics such as aspirin. It can somewhat relieve asthma attacks by dilating the bronchial airways.

Too much caffeine can produce restlessness, nausea, headache, tense muscles, sleep disturbances, and cardiac arrhythmias (irregular heartbeats). Because caffeine increases the production of stomach acid it may worsen ulcer symptoms or cause acid reflux ("heartburn"). Evening use of caffeine may disrupt sleep and cause insomnia.

Caffeine should be used with caution by people with heart disease and high blood pressure (hypertension), and by those suffering from the eye disease glaucoma. Caffeine medications should generally not be used in children. Many children are already consuming significant amounts of caffeine in drinks and food. In this connection, a nutritional concern is that children may choose fizzy drinks in preference to milk, thus getting "empty" calories at the expense of valuable nutrients.

Long-term effects

As already mentioned, some potentially harmful effects of coffee are recognized, particularly for people who should take few or no stimulants. Beyond this however, scientific studies of the effects of caffeine have in general failed to prove negative effects, although some have produced contradictory conclusions. An individual study may produce interesting results which may suggest fruitful directions for further research, but usually it is only when several independent studies confirm one another, and any contradictory results can be accounted for, that one can have reasonable confidence in the safety of a drug -- particularly an " optional" one like coffee.

Although caffeine does not fall into the class of "addictive" drugs, it may be habit-forming. Some people may experience headache, fatigue, irritability and nervousness when their daily intake of caffeine is quickly and substantially altered.

Such "withdrawal effects" may be responsible for confusing results in some studies. There are many complicating factors in long-term studies. One is the familiar "convergence of risk-factors" (e.g. that coffee-drinkers may be more likely to be smokers). Another is that many of the study subjects may deliberately change (or have previously changed) their consumption habits or behaviour, e.g. in response to discovering that they suffer from hypertension. There may also be significant differences in methods of coffee preparation between study populations, or over long periods of time.

Moderate caffeine consumption during pregnancy is generally considered safe. A study has not found any effect on low birth-weight or incidence of premature births. However, although it has been suggested that caffeine may stimulate milk production, cautious mothers may prefer to avoid such beverages during pregnancy or while breastfeeding.

Furthermore, a large study has not shown any connection of coffee or tea consumption with breast-cancer incidence. Osteoporosis is another condition which particularly affects women. Previous studies have suggested caffeine consumption as a risk-factor, but a recent analysis concludes that such an effect is probably not significant except in conditions of calcium-deficiency, which can be easily corrected.

There is even some actual positive news. The effect of caffeine on the risk of developing Parkinson's disease, which usually affects older people, has been found to be favourable for men. For women, previous results have been confusing; but a recent study suggests that a crucial factor may be the effect of hormone levels. Often caffeine may have a favourable effect against developing this disease; but when combined with hormone replacement therapy (HRT), it may have a negative one.

One study has found (for women) a strong inverse association between coffee intake and risk of suicide. However, even if confirmed, to determine whether this might be actual cause and effect is, as usual, a much more challenging problem.