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Health effects of caffeine

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Health effects of caffeine

Caffeine can be taken in tablet form

The health effects of caffeine have been extensively studied. Short term side effects such as headache, nausea, and anxiety have been shown as symptoms of mild caffeine consumption.[1] Caffeine competitively inhibits different adenosine receptors and their associated G protein to make a person feel alert.[2] A mild stimulant of the central nervous system, caffeine also stimulates cardiac muscle, relaxes smooth muscle, increases gastric secretions, and produces diuresis.[3]


Positive effects

  • Acetylcholine is associated with attention, concentration, learning, and memory but there is no conclusive evidence yet that caffeine has any effect on memory and cognitive function.[4]
  • Low doses of caffeine show increased alertness and decreased fatigue.[5]
  • Caffeine has been shown to increase the metabolic rate.[6][7][8]

Negative effects

  • Caffeine can increase blood pressure in non-habitual consumers.[9] High blood pressure is associated with an increase in strokes, and cerebral vascular disease, which in turn increase the risk of multi-infarct dementia.[9]
  • Caffeine may reduce control of fine motor movements (e.g. producing shaky hands)[5]
  • Caffeine can increase cortisol secretion, some tolerance is developed.[10]
  • Caffeine can contribute to increased insomnia and sleep latency.[5]
  • Caffeine is addictive.[5] Caffeine withdrawal can produce headache, fatigue and decreased alertness.
  • High doses of caffeine (300 mg or higher) can cause anxiety.[5]
  • High caffeine consumption accelerates bone loss at the spine in elderly postmenopausal women.[11]

Chemical properties

Two skeletal formulas: left - caffeine, right - adenosine.
Caffeine is an antagonist of adenosine.

Caffeine is a methylxanthine with similarity in molecular structure to the nucleotide adenosine. Methylxanthines or sometimes known as xanthines have a heterocyclic ring with nitrogen included in the ring structure; they are derived from amino acids, are basic in nature, and can generally form water soluble salts. Caffeine blocks the action of adenosine by acting as a competitive inhibitor for the A1 and A2a adenosine receptor.[12] With the initial absorption of caffeine occurring within 15 minutes the peak time of caffeine absorption is estimated at 45 minutes.[13] Although varying in many studies the half-life of caffeine has been found to be around 5.2-6.8 hours for adults.[14] However, the metabolic rate of caffeine absorption differs between each species.[13]

Toxicity and intoxication

Poisoning by caffeine
Classification and external resources
ICD-10 F15, T43.6
ICD-9 305.90, 969.71
eMedicine article/821863

Caffeine toxicity can present as a spectrum of clinical symptoms. Most of these originate in the central nervous and circulatory systems and can follow ingestion of 1g or more of caffeine. Insomnia, breathlessness and excitement progressing to mild delirium may be seen. Sensory disturbances, diuresis, tachycardia, extrasystoles, and elevated respirations as well as vomiting induced by potent gastric irritation can be present. Fatalities of caffeine poisoning are rare because the gastric irritation and vomiting develop before absorption of toxic amounts can occur. Normally, caffeine is rapidly and completely absorbed from the gastrointestinal tract with distribution in various tissues in approximate proportion to their water content. Convulsions result from the central stimulating effect with death caused by respiratory failure. Hyperglycemia and ketonuria associated with caffeine toxicity have been reported. These latter findings may be attributed to a stress reaction or the xanthine’s ability to mimic the metabolic effects of the cathecholamines including lipolysis, glycogenolysis and gluconeogenesis.[15] An acute overdose of caffeine usually in excess of about 300 milligrams, dependent on body weight and level of caffeine tolerance, can result in a state of central nervous system over-stimulation called caffeine intoxication,[16] or colloquially the "caffeine jitters". The median lethal dose (LD50) given orally, is 192 milligrams per kilogram in rats.[15] The LD50 of caffeine in humans is dependent on weight and individual sensitivity and estimated to be about 150 to 200 milligrams per kilogram of body mass, roughly 80 to 100 cups of coffee for an average adult taken within a limited time frame that is dependent on half-life. For additional information see: Caffeine.

Effects on different functions

Relationship between caffeine and adenosine

The predominant mechanism of action of caffeine is the antagonism of adenosine receptors. Adenosine is a locally released purine hormone that acts on different receptors (A1 and A2) that can increase or decrease cellular concentrations of cyclic AMP (cAMP). Adenosine inhibits adenyl cyclase via high affinity (A1) receptors and stimulates adenyl cyclase via low affinity (A2) receptors. Adenosine receptors are found throughout the body including the brain, in the cardiovascular, respiratory, renal and gastrointestinal system and in adipose tissue. Caffeine nonselectively blocks both adenosine receptors and competitively inhibits the actions of adenosine. Adenosine acts presynaptically to inhibit neuronal release of acetylcholine, norepinephrine, dopamine, gamma amino butyric acid and serotonin. Adenosine also reduces spontaneous firing of neurons in many regions of the brain which produces sedation and has anticonvulsant activity. Caffeine releases norepinephrine, dopamine, serotonin and glutamate in the brain and increases circulating catecholamines consistent with reversal of the inhibitory effects of adenosine on these systems.[17]

How caffeine wards off drowsiness

Caffeine and other methylxanthine derivatives are also used on newborns to treat apnea and correct irregular heartbeats. Caffeine stimulates the central nervous system first at the higher levels, resulting in increased alertness and wakefulness, faster and clearer flow of thought, increased focus, and better general body coordination, and later at the spinal cord level at higher doses.[18] Once inside the body, it has a complex chemistry, and acts through several mechanisms as described below:[19]

  • Increased calcium uptake activating metabolic pathways and neural activity
  • Activation of wake-promoting neurons
  • Release of neurotransmitters producing excitation
  • Release of glutamate producing anxiety
  • Increased blood flow to smooth muscles and vasoconstriction
  • Inhibition of dopamine reuptake
  • Inhibition of gamma-aminobutyric acid (GABA) and excitation of the CNS
  • Inhibition of sleep promoting neurons
  • Increased ATP production through cyclic adenosine monophosphate (cAMP) accumulation
  • Increased motor activity

Short-term effects


Caffeine increases cardiac arrhythmia (improper heart rate) by increasing stress hormone (e.g.adrenaline) secretions.[20] It has been shown there is an increase in brachial diastolic blood pressure, but not in brachial systolic blood pressure.[20] However, both aortic systolic and diastolic blood pressures increase significantly during caffeine consumption. It has been noted that long term consumption leads to increasing aortic systolic pressure which leads to chronic arterial stiffness.[21] The results of increasing blood pressure mostly contributes to blockage of Adenosine A1 and A2 receptors.[22] Since caffeine blocks adenosine A2 receptors which has vasodilatory function, blood vessels become less dilated. (i.e. vasoconstriction)[23] However, it is controversial whether caffeine consumption increases heart rate. Some research shows that caffeine has no influence on heart rate.[21][23]

Gastrointestinal distress

Caffeine can stimulate the secretion of these so-called stress hormones (such as epinephrine and norepinephrine), which can increase blood pressure. Moreover, stress hormones activate the body's "fight or flight" reactions, causing the body to redirect blood supply from the digestive system to muscles. In this way, decreased blood flow to the gastrointestinal tract will slow down the absorption rate and lead to indigestion.[24]

Moreover, the additional epinephrine increases the secretion of the main gastric hormone gastrin, which will speed up gastric peristalsis and hypersecretion of gastric acid.[25][26] Additional gastric acid will lead to acidic chyme going into the small intestine and cause intestinal injury.[24] Therefore, it is not recommended for ulcer patients to drink too much coffee.

Increased urination

Caffeine intake increases renal excretion of sodium and water. This is caused by both slightly increasing the glomerular filtration rate and inhibiting the tubular reabsorption of sodium and water. Although the ability for caffeine and theophylline to induce diuresis and natriuresis is well established, the mechanisms behind them are not well understood.[27] It has been suggested that inhibition of phosphodiesterases in the proximal tubule may contribute to the diuretic and natriuretic effects of methylxanthines. It has been noted that mice lacking the A1 receptor do not exhibit the diuresis and natriuresis typically elicited by the application of the methylxanthines caffeine or theophylline.[27]


Caffeine injections before exercise show no evidence of adverse effects such as dehydration or ion imbalance, and have no effect on short term exercise.[28] However, studies show that 5 mg/kg body weight of caffeine intake allows free fatty acids to mobilize faster, which in turn enhances the endurance performance during long term exercise. This only affects individuals who do not drink caffeine on a regular basis.[28] This means that if a 60 kg non-regular caffeine ingesting adult consumes 300 mg of caffeine, which is equivalent to 3 cups of regular coffee, before exercising, they will be able to exercise at the same intensity for a longer period of time.[28] As a side note, the International Olympic Committee since the summer of 2000, no longer treats caffeine as a banned substance.[28]

Caffeine is also known to have an effect on pain and perception of physical stress on human muscles.[29] A study done on human athletes demonstrated that caffeine consumers tend to rate their pain level lower relative to the placebo groups.[29] The mechanism for this reduced pain perception is often attributed to the A1 and A2 receptor that caffeine has an antagonistic effect against.[29] The decreased muscle pain is also cited as a possible reason for the improved endurance performance of some caffeine drinking athletes.[30]

Caffeine is also shown to have a slight positive effect on strength exercises such as bench press and squat, although the degree of the effects tends to depend on the consumer and their significance are often debated.[31] A consumption of a small caffeine dosage can improve maximum bench press by 0.8 kg on average for female weight lifters.[31] Another study done on male athletes demonstrates a slight 11% to 12% increase in maximal bench press and leg press.[32] It is unknown whether the increase in maximum weight is attributed to a direct effect from caffeine itself on the human muscles strength or from the stimulant’s known muscle pain reduction effect.[32]

Caffeine is also demonstrated to have an ergogenic effect on the neuromuscular function, peak force generation and muscular endurance.[33] Studies have shown that caffeine intakes can increase EMG rate, which indicates a positive effects on electrical activities in muscles.[33][34] The mechanism for the effects is often hypothesized to be the antagonistic effects of caffeine toward adenosine receptors, although some alternatives have been suggested. One of which is the stimulation of intracellular Ca2+ release within the body, which can increase tetanic tension.[33]


The US National Institutes of Health states "too much caffeine can make you restless, anxious, and irritable. It may also keep you from sleeping well and cause headaches, abnormal heart rhythms, or other problems. If you stop using caffeine, you could get withdrawal symptoms. Some people are more sensitive to the effects of caffeine than others. They should limit their use of caffeine. So should pregnant and nursing women."[35]

Four caffeine-induced disorders are recognized by the American Psychiatric Association (APA) including: caffeine intoxication, caffeine-induced sleep disorder, caffeine-induced anxiety disorder, and caffeine-related disorder not otherwise specified (NOS).[16] The DSM-IV defines someone with caffeine-induced sleep disorder, as an individual who regularly ingests high doses of caffeine sufficient to induce a significant disturbance in their sleep, sufficiently severe to warrant clinical attention.[16] As of 2010 the effect of caffeine on people with ADHD is not known.[36] Some studies have however found a modest protective against Alzheimer disease, but the evidence is inconclusive.[37][38][39]

Caffeine can have both positive and negative[40] effects on anxiety disorders.

1985, "It appears that anxiety disorder patients have increased caffeine sensitivity"[41] 1988, "panic patients have increased sensitivity to caffeine"[42] 1989, of 24 cases of GAD or Panic Disorder, 5 ceased caffeine intake and 1 significantly reduced caffeine intake; all 6 (25% of the sample group) saw much reduced symptoms many months later, and 5 took no further medication. In addition, 3 of the 6 took caffeine on one subsequent occasion and all 3 immediately saw anxiety symptoms occur.[43] 1992, "patients with GAD are abnormally sensitive to caffeine"[44] 1996, "there is clinical and experimental evidence that acute caffeine can exacerbate the effects of an anxiety-inducing situation, or worsen an existing anxiety disorder, especially panic disorder."[45] 2007, "Our data suggest that there is an association between panic attacks, no matter if associated with PD or MDP, and hyperreactivity to an oral caffeine challenge test."[46] 2010: 'Caffeine is anxiogenic [anxiety causing].'[47] 2011, Literature review, "The 8 studies all showed a positive association between caffeine and anxiogenic [anxiety causing] effects and/or panic disorder."[48]

At moderate and low doses the effect may depend on individual caffeine sensitivity.[49] Caffeine can cause anxiety symptoms in normal individuals, especially in vulnerable patients, like those with pre-existing anxiety disorders.[50] At high doses, typically greater than 300 mg, it can both cause and worsen anxiety[51] and rarely trigger mania and psychosis.

Caffeine withdrawal can also cause an increase in anxiety level.[36]

In moderate doses caffeine may reduce symptoms of depression and lower suicide risk.[36]

In moderate doses caffeine typically does not affect learning or memory.[52] It does however improve cognitive function in people who are fatigued, due to its effect on alertness.

Long-term effects

Type II diabetes

Although the relationship between tea caffeine and diabetes mellitus has been a topic under study for many years,[53] there remains insufficient evidence to conclude any effect of tea or caffeine on diabetes or any other disease, such as cancer.[54][55]

Negative effects on diabetics

Coffee caffeine may have a negative effect on diabetes patients because it appears to be capable of reducing insulin sensitivity and makes it difficult for patients with diabetes to control their blood glucose levels.[56] Studies have shown that caffeine consumption increases blood glucose, but diabetes medication decreases blood glucose levels, indicating that the two may counter-interact.[56]


Research has shown that there is no significant association between caffeine consumption and cancer, including cancer of the mouth, pharynx, esophagus, stomach, liver, or pancreas.[54][55][57]


Caffeine withdrawal
Classification and external resources
ICD-10 F15.1
ICD-9 292.0


Caffeine addiction leads to some levels of physical dependence. The most frequently seen withdrawal symptoms are headache and fatigue. Such symptoms affect consumers’ mood, but show no evidence of influencing ones’ performance. In prolonged caffeine drinkers, symptoms such as increased depression and anxiety, nausea, vomiting, physical pains and intense desire for beverages containing caffeine are also reported. The symptoms reflect consumers’ expectancy effects on caffeine to some degree.[5]

Withdrawal symptoms – including headache, irritability, inability to concentrate, drowsiness, insomnia, and pain in the stomach, upper body, and joints – may appear within 12 to 24 hours after discontinuation of caffeine intake, peak at roughly 48 hours, and usually last from 2 to 9 days.[58] Peer knowledge, support and interaction may aid withdrawal.[59][60][61]

Expectancy effects

Studies with placebo conditions have been done to demonstrate that the effects of caffeine depend greatly on the consumer’s expectations.[5] When heavy caffeine drinkers are led to believe that they are consuming beverages that contain caffeine, they tend to perform better regardless of the caffeine content within the sample that was given to them.

Effect of genetics on withdrawal symptoms

Gene polymorphism could be associated with caffeine withdrawal symptoms and beta 1and beta-2 play roles in caffeine withdrawal.[62] For example, compared to people with homozygous Gly16 allele, consumers with the heterozygote ADR beta-2 Gly16 Arg gene polymorphism have a higher chance of feeling fatigue after 48 hours of caffeine withdrawal.[62] It has been suspected that beta2- adrenoceptors are the main cause for this increase in mental fatigue symptoms.[62] Beta 2- adrenoceptors are receptors that regulate glycogenolysis, secret insulin and intramuscularly transport glucose that is used for cerebral and muscle activity.[62] Another example is given by the genes ADRbeta1 Gly16 Arg and CYP1A2-163A>C polymorphisms.[62] They are associated with the consumers’ mood swings and increased depression level.[62] Among subjects homozygous for the CYP1A2 allele, ADRbeta1 Gly389 allele carriers are reported to have a higher percentage of depression level increase when compared to Arg389 homozygotes subjects.[62] Adrenergic receptors, again, play a key role in this symptom, as altered norepinephrine (an adrenoceptor agonist) neurotransmission contribute to the etiology of depression.[62] This symptom is often seen in faster caffeine metabolizers, because caffeine effects diminish quicker in these consumers and provide them less opportunity to adapt to caffeine loss.[62]



Health Canada has not developed definitive advice for adolescents 13 and older because of insufficient data. Nonetheless, Health Canada suggests that daily caffeine intake for this age group be no more than 2.5 mg/kg body weight. This is because the maximum adult caffeine dose may not be appropriate for light weight adolescents or for younger adolescents who are still growing. The daily dose of 2.5 mg/kg body weight would not cause adverse health effects in the majority of adolescent caffeine consumers. This is a conservative suggestion since older and heavier weight adolescents may be able to consume adult doses of caffeine without suffering adverse effects. For the rest of the general population of healthy adults, Health Canada advises a daily intake of no more than 400 mg.[63]

According to the US-based Waverly Health Center, three 8 oz cups of coffee (about 250 milligrams of caffeine) per day is considered an average or moderate amount of caffeine; ten 8 oz cups of coffee per day is considered an excessive intake of caffeine.[64]

Pregnant women

Caffeine's potential impact on female fertility, and its precise impact on pregnancy, is still being studied, but (as with many other substances in these circumstances) caution and moderation is warranted in any case until further information is known. For women of childbearing age, Health Canada recommends a maximum daily caffeine intake of no more than 300 mg, or a little over two 8 oz (237 mL) cups of coffee.[63]


In healthy children, caffeine intake produces effects that are "modest and typically innocuous".[65] For children age 12 and under, Health Canada recommends a maximum daily caffeine intake of no more than 2.5 milligrams per kilogram of body weight. Based on average body weights of children, this translates to the following age-based intake limits:[63]
Age range Maximum recommended daily caffeine intake
4-6 45 mg (slightly more than in 12 oz of a typical soft drink)
7-9 62.5 mg
10-12 85 mg (about ½ cup of coffee)

Relatedly, one study found that caffeine can be used to treat hyperkinetic children.[66] The research found that 200–300 mg of caffeine has a similar effect to methylphenidate in treating hyperkinetic impulse disorder. Moreover, the caffeine treatment did not show the side-effects caused by methylphenidate.[66] Moderate caffeine intake has been reported to help children with ADHD with their focus time. Most ADHD children have a focus time of less than a minute at a time. Giving a child with ADHD a drink with at least 50 mg of caffeine, mainly coffee without added sugar can help increase this focus time. It also is reported to help calm or slow down the hyperactive behaviors.

Labeling requirements

It is a regulatory requirement that the label of most prepackaged foods must declare a list of ingredients, including food additives such as caffeine, in descending order of proportion. However, there is no regulatory provision for mandatory quantitative labeling of caffeine, (e.g., milligrams caffeine per stated serving size). As for "natural caffeine", there are a number of food ingredients that naturally contain caffeine. These ingredients must appear in food ingredient lists. However, as is the case for "food additive caffeine", there is no requirement to identify the quantitative amount of caffeine in composite foods containing ingredients that are natural sources of caffeine. While coffee or chocolate are broadly recognized as caffeine sources, some ingredients (e.g. guarana, yerba maté) are likely less recognized as caffeine sources. For these natural sources of caffeine, there is no regulatory provision requiring that a food label identify the presence of caffeine nor state the amount of caffeine present in the food.[67]

Effect of alcohol on caffeine

According to DSST, alcohol provides a reduction in performance and caffeine has a significant improvement in performance.[68] When alcohol and caffeine are consumed jointly, the effects produced by caffeine are affected, but the alcohol effects remain the same.[69] For example, when additional caffeine is added, the drug effect produced by alcohol is not reduced.[69] However, the jitteriness and alertness given by caffeine is decreased when additional alcohol is consumed.[69] Alcohol consumption alone reduces both inhibitory and activational aspects of behavioral control. Caffeine antagonizes the activational aspect of behavioral control, but has no effect on the inhibitory behavioral control.[70]

Alcohol is a GABA agonist and binds into the GABA receptors and decreases serotonin and acetylcholine activity, whereas caffeine increases acetylcholine and serotonin neurotransmission and decreases GABA.

Effect of orally administered birth control on caffeine

Consumption of caffeine while orally administering birth control can extend the half-life of caffeine; therefore, greater attention should be taken during caffeine consumption.[17]

See also


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