recreational drugs

The commonest recreational drugs in our society are alcohol and nicotine. More than 80 per cent of adults admit to using alcohol occasionally, and despite the health hazards associated with tobacco more than a third continue to smoke. Third in popularity (although illegal) is cannabis (known as marijuana in the USA). Cannabis is tried at least once by most teenagers and used regularly by millions of young people. Much smaller numbers of people are regular users of the so-called 'hard drugs', cocaine, heroin, and amphetamines.

People take recreational drugs because they find their effects on the brain pleasurable. The various drugs differ in their primary actions in the brain. Some drugs act because they mimic the actions of naturally occurring chemicals associated with normal physiological brain mechanisms. Thus, heroin is recognized in the brain by specific receptor proteins that normally recognize and react to the endorphins — a series of naturally occurring chemicals in the brain that play roles in signalling pain and pleasure. The active chemical in cannabis, delta-9-tetrahydrocannabinol (THC), is recognized by a cannabinoid receptor which is normally targeted by the naturally occurring cannabis-like chemical anandamide in the brain. Cocaine acts by blocking the inactivation of the brain chemical dopamine after its release in the brain — thus enhancing the effects of dopamine — and amphetamines act similarly to increase the availability of dopamine in the brain by stimulating its release. The intoxicants alcohol and THC act by both enhancing the actions of the inhibitory chemical messenger GABA (gamma-amino butyric acid) in the brain, and simultaneously dampening the actions of a key excitatory chemical, l-glutamate. The psychedelics (d-LSD, mescaline, psilocin, 'magic mushroom') have very different actions. These are not drugs of addiction, and the sensations they induce represent highly unusual states of consciousness. Much less is known about the precise circuits that are activated by these drugs, although a common mechanism of action appears to be activation of receptors that normally recognize the brain chemical messenger serotonin.

Neuroscientists have found in recent years that although the various individual drugs act on different primary mechanisms in the brain there are certain final common pathways that are triggered by many of the recreational drugs. In particular pathways that use the brain chemical dopamine in the forebrain are activated in a similar way by alcohol, nicotine, heroin, amphetamines, cocaine, and THC. These are brain mechanisms involved in pleasure and reward that are important for the normal functioning of the brain in, for example, learning and memory — in which reward for correct responses plays a key role. In effect drugs of abuse 'hijack' these brain mechanisms, i.e. drug-induced activation of the brain mechanisms normally associated with learning and memory triggers drug-seeking behaviour, dependence, and addiction. In establishing addiction it seems to be important that the taking of the drug is associated with an immediately pleasurable sensation if addiction is to be learned rapidly. Thus, drugs that can be injected or smoked, which deliver the active chemical to the brain very rapidly, tend to be most addictive. For example, injected or smoked heroin, insufflated or smoked cocaine, smoked cannabis, and smoked or injected methamphetamine ('speed') are all readily addictive, whereas the same drugs taken by mouth are absorbed much more slowly and are less likely to lead to addiction. Children treated with amphetamines (e.g. Ritalin) for 'attention deficit hyperactivity disorder' (ADHD), for example, rarely become addicted — but injected or smoked amphetamines can be highly addictive.

Addiction to alcohol, heroin, and cocaine is often associated with physical signs of withdrawal if the drug is removed. These include severe gastrointestinal disruption, pain, insomnia, and possibly life-threatening convulsions. Addiction to nicotine or THC is not associated with physical signs of withdrawal but there may be intense psychological distress if the drug is withdrawn. Addiction is also associated with a craving for the drug, which together with the desire to avoid unpleasant withdrawal effects motivates continuing drug-taking behaviour. Addiction per se is not necessarily harmful — many coffee drinkers are mildly addicted to caffeine but it does them no personal harm and does not damage society. But addiction to recreational drugs can damage both the addicts and society. The long-term consumption of alcohol and tobacco is associated with the risk of life-threatening illnesses (e.g. cirrhosis of the liver, lung cancer) — and these represent some of the largest preventable causes of death. Smoking cannabis carries health risks — particularly to the lungs — and the heavy use of cannabis by young people also appears to be associated with an increased risk of developing schizophrenia-like illness later in life. Addiction to 'hard drugs' is frequently associated with criminality, as both addicts and suppliers operate in a criminal underworld in which addicts may be driven to crime to fund their drug habit. Not all recreational drugs are equally addictive. Although a proportion (perhaps as high as 10 per cent) of regular cannabis users become psychologically dependent on the drug, most are able to stop using cannabis by the time they reach their 30s. In contrast most cigarette smokers find it very difficult to quit. Although it is difficult to measure the 'addictiveness' of drugs, most pharmacologists have a mental list ranking the recreational drugs in this way. At the top of my list would be nicotine, followed by cocaine, heroin, amphetamines, cannabis, alcohol, and caffeine.

Attempts to control the abuse of recreational drugs by punishing the users as criminals have not been successful in curbing the increasing problem that drug abuse poses. More emphasis is increasingly being placed on the medical model, which attempts to treat addiction as a medical problem. Unfortunately few treatments have been found to be effective — although a combination of behavioural therapy and medicines shows most promise. In terms of medicines, the most effective approach that has been found so far is to replace the addictive drug with a less harmful form of the same type of chemical. Thus, cigarette smokers use nicotine-containing chewing gum or a skin patch loaded with nicotine to obtain nicotine rather then inhaling harmful tobacco smoke. Heroin addicts may be treated with methadone — a drug that targets the same brain receptors but is taken by mouth and acts slowly to reduce craving. New approaches to the treatment of drug addiction are badly needed but so far progress has been slow.

(Published 2004)

— Les Iverson

Bibliography
• Everitt, B. J., and Dickinson, A. (2001). 'The neuropsychological basis of addiction behaviour'. Brain Research Review, 36.
• Iversen, L. L. (2001). A Very Short Introduction to Drugs.
• Koob, G. F. (2000). 'Neurobiology of addiction: towards the development of new therapies'. Annals of the New York Academy of Science, 909.
• Lingford-Hughes, A. R. et al. (2003). 'Addiction'. British Medical Bulletin, 65.
• Nestler, E. J. (2002). 'From neurobiology to treatment: progress against addiction'. Nature Neuroscience, Suppl. (Nov.).

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