The adverse health effects of cannabis and other drugs Comparisons of Cannabis with Other Drugs

How do these potential harms compare with those of other psychoactive substances in non-medical use?

One important dimension of dangerousness or harm is the likelihood of a fatal overdose (See column 1 of Table 2.1) (Gable, 2004). The "safety ratio" is the ratio between "the usual effective dose for non-medical purposes" and the usual lethal dose. Cannabis was in the lowest-risk group on this scale, along with other substances that have a ratio of 100 or higher.

Another dimension of dangerousness is the level of intoxication produced by the substance. This is influenced by the dose used, and the set and setting in which it is consumed. Nonetheless, there are differences in the propensity of different psychoactive substances to intoxicate users. The second column of Table 2.1 shows rankings made by Henningfield and Benowitz on this dimension (Hilts, 1994). Cannabis was ranked as more intoxicating than tobacco, but less so than alcohol, cocaine and heroin.

Table 2.1. Ratings on dimensions of danger

Safety

Intoxicating

Dependence

Potential

Degree of

ratio

effect

(how hard to

addictiveness

psychic

(Gable,

(Hilts, 1994)

quit)

(Strategy Unit,

dependence

2004)

(Hilts, 1994)

2005)

(Roques, 1999)

Cannabis

>1000 sm

4th highest

Lowest

**

Weak

MDMA

16 or

Nr

Nr

**

?

Stimulants

10 or

Nr

Nr

***

Middling

Tobacco

Nr

5th highest

Highest

***

Very strong

Alcohol

10 or

Highest

4th highest

***

Very strong

Cocaine

15 in

3rd highest

3rd highest

***

Strong but intermittent

Heroin

6 iv

2nd highest

2nd highest

*****

Very strong

nr = not rated; sm = smoked; or = oral; in = intranasal; iv = intravenous Safety ratio = (usual effective dose for nonmedical purposes)/(usual lethal dose)

nr = not rated; sm = smoked; or = oral; in = intranasal; iv = intravenous Safety ratio = (usual effective dose for nonmedical purposes)/(usual lethal dose)

Ratings of the dependence potential or addictiveness of different substances (e.g. Hilts, 1994) compare drugs on withdrawal, tolerance, reinforcement and dependence. The report of the UK Prime Minister's Strategy Unit (2005) rated drugs on their "potential addictiveness" and a French committee chaired by Bernard Roques (1999) rated them on "psychic dependence" (see last three columns of Table 2.1). Although there is some disagreement in the rankings for other drugs, each placed cannabis at the lowest level for the substances in the table.

The Roques committee took a more global approach to rating dangerousness. Table 2.2 shows the committee's rankings on "Toxicité générale" (general toxicity) and "Dangerosité sociale" (social dangerousness). In the Roques report, "toxicity" included long-term health effects such as cancer and liver disease, infections, other consequences of use, and acute effects represented by the safety ratio. The concept of social dangerousness focused on "states of comportment which can generate very aggressive and uncontrolled conduct ... induced by the product or varied disorders (fights, robberies, crimes ...) in order to obtain it and risks for the

Table 2.2 Ratings on global dimensions of dangerousness (Roques, 1999)

General toxicity

Social dangerousness

Cannabis

Very weak

Weak

Benzodiazepines (Valium)

Very weak

Weak (except when driving)

MDMA/Ecstasy

Possibly very strong

Weak (?)

Stimulants

Strong

Weak (possible exceptions)

Tobacco

Very strong

None

Alcohol

Strong

Strong

Cocaine

Strong

Very strong

Heroin

Strong (except therapeutic use of opiates)

Very strong

user or others, for example in the case of driving a vehicle" (Roques, 1999, p. 296; original in French). The Roques ratings on general toxicity are compatible with the safety ratios of Gable (2004), and the social dangerousness ratings are similar to Henningfield and Benowitz's ratings of intoxicating effect (Hilts, 1994). Cannabis is ranked "weak" on general toxicity, and "very weak" on social dangerousness.

Hall et al. (1999a) compared four substances on whether there was "important effect" or a "less common or less well-established effect" on each of 11 dimensions (Table 2.3). According to these rankings, alcohol clearly has the greatest potential for harm while cannabis had the lowest number of asterisks among the four substances rated.

Nutt et al. (2007) used the ratings of experts to arrive at a global rating of the comparative harm of different drugs. They identified three main factors that determined the harms of different drugs: (i) the physical harm to the individual user; (ii) the tendency of the drug to induce dependence; and (iii) the effect of drug use on families, communities, and society. Within these categories, they recognised three components, to create a 9-category "matrix of harm".

Table 2.3. Summary of adverse effects on health for heavy users of the most harmful common form of each of four drugs (Hall et al., 1999a)

Cannabis

Tobacco

Heroin

Alcohol

Traffic and other accidents

*

*

**

Violence and suicide

**

Overdose death

**

*

HIV and liver infections

**

*

Liver cirrhosis

**

Heart disease

**

*

Respiratory diseases

*

**

Cancers

*

**

*

Mental illness

*

**

Dependence/Addiction

**

**

**

**

Lasting effects on the foetus

*

*

*

**

** = important effect; * = less common or less well-established effect

** = important effect; * = less common or less well-established effect

Physical harms were split into "acute", "chronic", and "intravenous" harm. Dependence was split into "intensity of pleasure", "psychological dependence" and "physical dependence". Social harms were split into "intoxication", "other social harms" and "health care costs". Expert panels of psychiatrists, pharmacologists, and addiction specialists were asked to give scores, from zero to three, for each category of harm for 20 different drugs. Cannabis was rated at eleventh most harmful out of 20 substances. Heroin and cocaine were rated the most harmful, while alcohol and tobacco, the benzodiazepines and amphetamines were rated more harmful than cannabis. Cannabis was scored well below the midpoint of scores on most dimensions. It scored above the midpoint only on intensity of pleasure, intoxication, and chronic physical harm.

The Public Health Impact Of Cannabis Use

Comparisons of the public health burden of cannabis with those of alcohol, tobacco and other illicit drugs have rarely been attempted because of dearth of evidence on impact on mortality and morbidity (Hall et al., 2008 in press; Hall et al., 2006). One of the earliest attempts (Hall, 1995) made a qualitative assessment that identified the most important public health impacts of cannabis use "in order of approximate public health importance" as: motor vehicle accidents; cannabis dependence; respiratory disease; precipitation and exacerbation of schizophrenia in vulnerable individuals; low birth weight babies; and subtle cognitive impairment.

The most recent estimate of the contribution of illicit drugs to the global burden of disease (BOD) confined itself to estimating the contribution of illicit opioid use, because these drugs had the best epidemiological evidence of adverse impact on mortality. Studies estimating the economic costs of alcohol, tobacco and illicit drugs have often not disaggregated the effects of cannabis from those of opioids (e.g. Collins & Lapsley, 2007). One recent study that did disaggregate cannabis (Rehm et al., 2007) only counted morbidity that could be directly attributed to cannabis via a diagnostic code, namely, episodes of hospital care for cannabis dependence.

A recent Australian study did make a more serious attempt to estimate the contribution that cannabis use made to the burden of disease (BOD) in Australia

(Begg et al., 2007). This study included estimates of disability due to cannabis dependence and cannabis-related psychoses, and it also attributed small proportions of MVA deaths and suicides to cannabis use. It estimated that cannabis was responsible for 0.2% of total disease burden. This comprised 10% of the BOD attributable to the use of all illicit drugs (2.0%), a similar proportion of that due to alcohol (2.3%) but a small fraction of that due to tobacco use (7.8%). Even allowing for under-estimation, the contribution of cannabis to BOD on current patterns of use was very modest in a country with one of the highest prevalences worldwide of cannabis use in the late 1990s (UNODC, 2006). Even so, the contribution of cannabis might well rise somewhat if it was as freely available, as heavily promoted and as widely used as alcohol and tobacco are now.

Summary

The acute adverse effects of cannabis use include anxiety and panic, especially in naive users, and an increased risk of accident if a person drives a motor vehicle while intoxicated with cannabis. Women who smoke during pregnancy increase their risk of giving birth to a low-birth weight baby.

The most probable adverse health effects of chronic cannabis use are increased risks of: a cannabis dependence syndrome; chronic bronchitis and impaired respiratory function in regular smokers; increased risk of cardiovascular disease in older adults who continue to smoke into middle age; respiratory cancers in very long-term daily smokers; and psychotic symptoms and disorders in heavy users - especially those with a pre-existing history of such symptoms, a family history of such disorders, or who begin use in their early teens. Among the most probable adverse psychosocial effects among adolescents who initiate early are an increased risk of cannabis dependence and impaired educational attainment. Regular adolescent cannabis users have a higher risk of using other illicit drugs, although the explanation of this relationship and its implications remain contentious.

The public health impact of contemporary patterns of cannabis use are modest by comparison with those of other illicit drugs (such as the opioids) or with alcohol. In the former case this reflects the absence of fatal overdose risk from cannabis. In the latter case, it reflects the much lower risks of death from cannabis-than alcohol-impaired driving, fewer adverse effects on health, lower rates of regular use to intoxication for cannabis than for alcohol, and the lower rate of persistence of cannabis use into older adulthood.

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