CAFFEINE AND HABITUATION
Caffeine has been consumed by human beings for centuries and its consumption is common in many different cultures. This makes caffeine the most widely consumed pharmacologically active substance, as it is a stimulant of the central nervous system. However, its effects on alertness are valuable in many everyday situations. For example, there is growing evidence that caffeine intake from coffee can increase alertness in car drivers and hence reduce accidents. Sleep-related car accidents show peaks in the early morning and the early afternoon. An intake of 150 mg caffeine in coffee has been shown to significantly reduce driving impairments, subjective sleepiness and electroencephalographic (EEG) activity indicating drowsiness in early afternoon driving tests. A subsequent study showed that 200 mg caffeine in coffee significantly reduced driving incidents, subjective sleepiness and EEG activity in early morning driving tests. Caffeine is more effective than alternative strategies for combating fatigue including cold air blown on the face or the use of a radio or tape player.
Despite these valuable effects in everyday life, caffeine has been described as “a potential drug of abuse” and it has also been suggested that caffeine is a “model drug of abuse”. The evidence for caffeine abuse, dependence and withdrawal has been reviewed and the possibility that caffeine withdrawal, although not caffeine abuse or dependence, should be added to diagnostic manuals has been discussed.
However, regulatory agencies impose no restrictions on the use of caffeine. The recent diagnostic manuals from the World Health Organisation (WHO) and the American Psychiatric Association (APA) have set criteria for dependence. A diagnosis of dependence according to the International Classification of Diseases (ICD)-10 system for mental and behavioural disorders requires that three out of six criteria are met. The WHO has stated that “There is no evidence whatsoever that caffeine use has even remotely comparable physical and social consequences which are associated with serious drugs of abuse”. The APA have proposed seven similar criteria, three of which have to be met, in the Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV). According to these criteria, the main factors to consider are withdrawal, tolerance, reinforcement and dependence.
WITHDRAWAL, TOLERANCE, REINFORCEMENT AND DEPENDENCE
The evidence for caffeine withdrawal, tolerance, reinforcement and dependence has been reviewed recently. It was concluded that “A withdrawal syndrome to caffeine has been described which does not seem to relate to the quantity of caffeine ingested daily” and that “Withdrawal symptoms, although relatively limited with respect to severity, do occur, and may contribute to maintenance of caffeine consumption”. “In humans, tolerance to some subjective effects of caffeine as well as partial tolerance to sleep seems to occur, at least in some individuals”. “In humans, the reinforcing stimuli functions of caffeine are limited to low or rather moderate doses that are usually present in a classical serving of coffee or soft drink” and “Caffeine has weak reinforcing properties, but with little or no evidence for upward dose adjustment possible because of the adverse effects of higher doses”. These two reviews suggest that caffeine meets some but not enough of the criteria for a diagnosis of dependence.
The authors of a review paper published in 2004 argue that caffeine should be added to the list of substances for inclusion in any new edition of the US Diagnostic & Statistical Manual of Mental Disorders, however, many of the studies included in the appendix of their review have failed to support claims that caffeine withdrawal is a consistent syndrome at all.
Other considerations also argue against caffeine dependence. It is known that the classic drugs of abuse such as amphetamines, cocaine and nicotine stimulate the release of dopamine in the shell of the nucleus accumbens, the key structure in the brain for reward, motivation and addiction. However, caffeine has no effect on the shell of the nucleus accumbens. Caffeine is taken orally and spread throughout the day unlike drugs of abuse which are commonly taken intravenously or by inhalation. This results in a delay in caffeine absorption that reduces the risk of dependence.
Research has been ongoing into this area of health and a comprehensive review published in 2006 took a common sense approach to addiciton and withdrawal. In thier abstract they state 'The common-sense use of the term addiciton is that regular consumption is irresistable and that it creates problems. Caffeine use does not fit this profile. Its intake does no harm to the individual or to society and its users are not compelled to consume it. Though cessation of regular use may result in symptoms such as headache and lethargy, these are easily and reliably reversed by ingestion of caffeine. Some have agreed that continued caffeine use is an attempt to suppress low grade withdrawal symptoms such as sleepiness and lethargy. In some moderate users, this is possible; however, in experimental contexts, the phenomenon is too inconsistent to constitute a reliably valid syndrome'. From thier review of the available data these authors conclude 'Thus, caffeine use meets neither the common sense nor the scientific definitions of an addictive substance'.