Apr 6, 2014
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Physical or psychological reliance on a substance or an individual. A baby is naturally dependent on its parents, but as the child develops, this dependence lessens. Some adults, however, remain partly dependent, making abnormal demands for admiration, love and help from parents, relatives and others.

The dependence that most concerns modern society is one in which individuals become dependent on or addicted to certain substances such as alcohol, drugs, tobacco (nicotine), caffeine and solvents. This is often called substance abuse. Some people become addicted to certain foods or activities: examples of the latter include gambling, computer games and use of the Internet.

The 28th report of the World Health Organisation Expert Committee on Drug Dependence in 1993 defined drug dependence as: ‘A cluster of physiological, behavioural and cognitive phenomena of variable intensity, in which the use of a psychoactive drug (or drugs) takes on a high priority. The necessary descriptive characteristics are preoccupation with a desire to obtain and take the drug and persistent drug-seeking behaviour. Psychological dependence occurs when the substance abuser craves the drug’s desirable effects. Physical dependence occurs when the user has to continue taking the drug to avoid distressing withdrawal or abstinence symptoms. Thus, determinants and the problematic consequences of drug dependence may be biological, psychological or social and usually interact.’

Different drugs cause different rates of dependence: TOBACCO is the most common substance of addiction; HEROIN and COCAINE cause high rates of addiction; whereas ALCOHOL is much lower, and CANNABIS lower again. Smoking in the western world reached a peak after World War II with almost 80 per cent of the male population smoking. The reports on the link between smoking and cancer in the early 1960s resulted in a decline that has continued so that only around a quarter of the adult populations of the UK and USA smokes. Globally, tobacco consumption continues to grow, particularly in the developing world with multinational tobacco companies marketing their products aggressively.

Accurate figures for illegal drug-taking are hard to obtain, but probably approximately 4 per cent of the population is dependent on alcohol and 2 per cent on other drugs, both legal and illegal, at any one time in western countries.

How does dependence occur? More than 40 distinct theories or models of drug misuse have been put forward. One is that the individual consumes drugs to cope with personal problems or difficulties in relations with others. The other main model emphasises environmental influences such as drug availability, environmental pressures to consume drugs, and sociocultural influences such as peer pressure.

By contrast to these models of why people misuse drugs, models of compulsive drug use – where individuals have a compulsive addiction

– have been amenable to testing in the laboratory. Studies at cellular and nerve-receptor levels are attempting to identify mechanisms of tolerance and dependence for several substances. Classical behaviour theory is a key model for understanding drug dependence. This and current laboratory studies are being used to explain the reinforcing nature of dependent substances and are helping to provide an explanatory framework for dependence. Drug consumption is a learned form of behaviour. Numerous investigators have used conditioning theories to study why people misuse drugs. Laboratory studies are now locating the ‘reward pathways’ in the brain for opiates and stimulants where positive reinforcing mechanisms involve particular sectors of the brain. There is a consensus among experts in addiction that addictive behaviour is amenable to effective treatment, and that the extent to which an addict complies with treatment makes it possible to predict a positive outcome. But there is a long way to go before the mechanisms of drug addiction are properly understood or ways of treating it generally agreed.

Effects of drugs Cannabis, derived from the plant Cannabis sativa, is a widely used recreational drug. Its two main forms are marijuana, which comes from the dried leaves, and hashish which comes from the resin. Cannabis may be used in food and drink but is usually smoked in cigarettes to induce relaxation and a feeling of well-being. Heavy use can cause apathy and vagueness and may even cause psychosis. Whether or not cannabis leads people to using harder drugs is arguable, and a national debate is underway on whether its use should be legalised for medicinal use. Cannabis may alleviate the symptoms of some disorders – for example, MULTIPLE SCLEROSIS (MS) – and there are calls to allow the substance to be classified as a prescribable drug.

About one in ten of Britain’s teenagers misuses volatile substances such as toluene at some time, but only about one in 40 does so regularly. These substances are given off by certain glues, solvents, varnishes, and liquid fuels, all of which can be bought cheaply in shops, although their sale to children under 16 is illegal. They are often inhaled from plastic bags held over the nose and mouth. Central-nervous-system excitation, with euphoria and disinhibition, is followed by depression and lethargy. Unpleasant effects include facial rash, nausea and vomiting, tremor, dizziness, and clumsiness. Death from COMA and acute cardiac toxicity is a serious risk. Chronic heavy use can cause peripheral neuropathy and irreversible cerebellar damage. (See SOLVENT ABUSE (MISUSE).)

The hallucinogenic or psychedelic drugs include LYSERGIC ACID DIETHYLAMIDE (LSD) or acid, magic mushrooms, ecstasy (MDMA), and phencyclidine (PCP or ‘angel’ dust, mainly used in the USA). These drugs have no medicinal uses. Taken by mouth, they produce vivid ‘trips’, with heightened emotions and perceptions and sometimes with hallucinations. They are not physically addictive but can cause nightmarish bad trips during use and flashbacks (vivid reruns of trips) after use, and can probably trigger psychosis and even death, especially if drugs are mixed or taken with alcohol.

Stimulant drugs such as amphetamine and cocaine act like adrenaline and speed up the central nervous system, making the user feel confident, energetic, and powerful for several hours. They can also cause severe insomnia, anxiety, paranoia, psychosis, and even sudden death due to convulsions or tachycardia. Depression may occur on withdrawal of these drugs, and in some users this is sufficiently deterrent to cause psychological dependence. Amphetamine (‘speed’) is mainly synthesised illegally and may be eaten, sniffed, or injected. Related drugs, such as dexamphetamine sulphate (Dexedrine), are prescribed pills that enter the black market. ECSTASY is another amphetamine derivative that has become a popular recreational drug; it may have fatal allergic effects. Cocaine and related drugs are used in medicine as local anaesthetics. Illegal supplies of cocaine (‘snow’ or ‘ice’) and its derivative, ‘crack’, come mainly from South America, where they are made from the plant Erythroxylon coca. Cocaine is usually sniffed (‘snorted’) or rubbed into the gums; crack is burnt and inhaled.

Opiate drugs are derived from the opium poppy, Papaver somniferum. They are described as narcotic because they induce sleep. Their main medical use is as potent oral or injectable analgesics such as MORPHINE, DIAMORPHINE, PETHIDINE HYDROCHLORIDE, and CODEINE. The commonest illegal opiate is heroin, a powdered form of diamorphine that may be smoked, sniffed, or injected to induce euphoria and drowsiness. Regular opiate misuse leads to tolerance (the need to take ever larger doses to achieve the same effect) and marked dependence. A less addictive oral opiate, METHADONE HYDROCHLORIDE, can be prescribed as a substitute that is easier to withdraw.

Some 75,000–150,000 Britons now misuse opiates and other drugs intravenously, and pose a huge public-health problem because injections with shared dirty needles can carry the blood-borne viruses that cause AIDS/HIV and HEPATITIS B. Many clinics now operate schemes to exchange old needles for clean ones, free of charge. Many addicts are often socially disruptive.


(See ALCOHOL and TOBACCO for detailed entries on those subjects.)

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Medical Dictionary

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