Effects Of Caffeine

Caffeine can affect the body in a number of ways.

The Effects Of Caffeine On The Body

As a stimulant, caffeine affects the body's metabolism.


  • stimulates the CNS (central nervous system)
  • increases blood flow to the kidneys, increasing urination and subsequent risk of dehydration
  • causes the release of fatty acids from fatty, adipose tissue
  • increases alertness
  • delays fatigue and raises energy levels

Although caffeine is not stored in the body, the effects of caffeine are long lasting. It can take hours for the effects to dissipate and the caffeine eliminated from the body.

Caffeine And Sleep

Caffeine can disrupt sleep. By taking caffeine less than 8 hours before bed, this stimulant drug can significantly impact on the quality of sleep making it more difficult to fall asleep. As caffeine can also increase urination, there may be more trips to the toilet throughout the night.

Disrupted sleep can cause a reduction in daytime alertness and concentration. One consequence is that a person may consume more caffeine (e.g. by drinking coffee) to stay alert.

The caffeine-sleep cycle can be reduced by cutting down on caffeine intake and avoiding caffeine many hours before bedtime.

Negative Effects Of Caffeine

Symptoms Of Too Much Caffeine

High doses of caffeine (500mg + daily doses) may lead to any of the following negative effects:

  • restlessness
  • anxiety – including feeling jittery (feelings of nervous unease)
  • irritability
  • sleep disturbance – (see also, 'caffeine and sleep' above)
  • abnormal heart rhythms – irregular or fast heart beat
  • headaches
  • gastrointestinal disturbance – e.g. stomach upset
  • nausea
  • muscle tremors

Caffeine Sensitivity

There may be differences in the sensitivity of caffeine between people. For example, in some people just one cup of coffee may bring on some of the unwanted negative effects of caffeine. See also, 'susceptibility to side effects' below.

Caffeine & Pregnancy

Pregnant women and those breastfeeding should avoid taking caffeine.

Caffeine & Drug / Medicine Interaction

Caffeine may interact with certain drugs. Negative drugs interactions from caffeine include:

Theophylline – blood concentrations of theophylline may be increased if taken alongside caffeine. This may lead to unpleasant effects including:

  • heart palpitations
  • vomiting
  • nausea

Some antibiotics – antibiotics such as ciprofloxacin and norfloxacin can disrupt the breakdown of caffeine. This causes caffeine to remain in the body for longer and possible negative side effects of caffeine may be increased.

Caffeine & Coronary Heart Disease

Coronary Heart Disease – at present there is no conclusive link between caffeine intake from coffee and risk of coronary heart disease (CHD). This link is still being researched. Results from current studies are conflicting and may be due to differences in the design of the studies. As a guide, 1-2 cups of coffee per day does not appear to be harmful [1].

Susceptibility To The Side Effects Of Caffeine

There are a number of factors which can influence an individuals susceptibility to the negative effects of caffeine.

  • Tolerance – people who regularly consume caffeine can build up tolerance to this drug.
    Tolerance can influence how a person reacts to caffeine and may also influence the severity of withdrawal symptoms should they no longer consume caffeine.
  • Gender – men may be more susceptible to the negative effects of caffeine than women
  • Weight – people with a lower body mass index (BMI), classed as underweight may have a higher sensitivity to caffeine than those classed as being over weight.
  • Age
  • If the person is a smoker
  • Current medication a person is taking
  • Levels of stress
  • Existing health conditions – e.g. a person suffering from an anxiety related disorder may be more susceptible to the effects of caffeine.