Relative risks of abused drugs

This is a critical issue in relation to directing legal as well as medical inputs into drug abuse. There are four main factors which have to be taken into account in determining relative risk:

• risk of the drug itself

• extent drug controls behaviour (addictiveness)

The risks due to the route have been covered above. The risks of the drugs themselves are determined by standard tests and clinical experience and can be encapsulated in concepts such as the therapeutic index. This is the ratio of toxic dose to therapeutic (or usual) dose. The ratio is very low for heroin and similar opiates, for cocaine especially crack, and for intravenous temazepam and oral ecstasy. It is quite high for psychedelics, cannabis, benzodiazepines, and orally used stimulants such as amphetamines. Another important consideration is the health complications of long-term use which by and large reflects the therapeutic index. An exception to this is the opiates, which, provided sterile administration is used, are thought to have little detrimental effect, even when used chronically and intravenously. Chronic cocaine can lead to cardiac damage, and heavy cannabis smoking causes precancerous change in the same way as tobacco smoking, as well as causing greater levels of chronic bronchitis.

The degree of control over behaviour the drug elicits is a major factor in drug dependence, and is the closest concept to addictiveness. Although the route of administration is another critical variable, we can make some reasonable generalizations. Strong opiates and cocaine are the most addictive, being in the same class as nicotine. The benzodiazepines, ecstasy, and psychedelics are the least addictive, and are significantly less addictive than alcohol.

There are three main factors contributing to drugs gaining control over behaviour, all of which affect the ease with which a drug may be stopped. The first is the pleasure a drug produces—the positive drive for use (pleasure giving and seeking). The others both involve the pain of abstinence—withdrawal in both physical and psychological terms—which leads to drug use to relieve it (discomfort escape). The pattern of drug use during an addiction career generally begins with the quest for pleasure and progressively evolves into the escape from withdrawal pain as neuroadaptive processes develop. In this context it may be thought that withdrawal discomfort is best limited to symptoms with a clear physical symptomatology, i.e. the autonomic symptoms indicative of physical dependence. But in terms of addictiveness, psychological withdrawal may in fact be more important than physical withdrawal. This is illustrated by the finding that those dependent on opiates for medical reasons, although physically dependent, experience little craving and risk of relapse once detoxified, provided the reason for being on the opiate resolves. The ease of stopping the drug thus depends on both the physical and psychological withdrawal symptoms, as well as the ability of the drug to provide positive reinforcement.

It is possible to provide rough guides for these three processes for each drug so that the overall addictiveness potential can be gauged ( Tab.l.e.3). For completeness, the main licit drugs are also shown as well as another highly motivated behaviour which can produce a state of addiction/dependence, i.e. gambling.

Table 3 Addictiveness of various agents

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