Consecutive Days

Figure 2

The Termination Effects of Daily Caffeine Consumption

SOURCE: Griffiths et al. (1990). Low-dose caffeine physical dependence in humans.

(e.g., improvements in vigilance and reaction time). Higher doses produce reports of nervousness and anxiousness, measurable disturbances in sleep, and increases in tremor. Very high doses can produce convulsions.

Caffeine's cardiovascular effects are variable and depend on dose, route of administration, rate of administration, and history of caffeine consumption. Caffeine doses between 250 and 350 milligrams can produce small increases in blood pressure in caffeine-abstinent adults. Daily caffeine administration, however, produces tolerance to these cardiovascular effects within several days; thus comparable caffeine doses do not reliably affect blood pressure of regular caffeine consumers.

High caffeine doses can produce a rapid heartbeat (tachycardia) and in rare cases irregularities in heartbeat (cardiac arrhythmia). Caffeine's effects on peripheral blood flow and vascular resistance are variable. In contrast, caffeine appears to increase cerebrovascular resistance and decrease cerebral blood flow.

Moderate doses of caffeine can increase respiratory rate in caffeine-abstinent adults. Caffeine also relaxes the smooth muscles of the bronchi. Because of caffeine effects on respiration, it has been used to treat asthma, chronic obstructive pulmonary disease, and neonatal apnea (transient cessation of respiration in newborns).

Moderate doses of caffeine can act on the kidney to produce diuretic effects that diminish after chronic dosing. Caffeine has a variety of effects on the gastrointestinal system, particularly the stimulation of acid secretion. These effects can contribute to digestive upset and to ulcers of the gastrointestinal system.

Caffeine increases the concentration of free fatty acids in plasma and increases the basal metabolic rate.

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