CANNABIS: a health perspective and research agenda

World Health Organization (WHO)
May, 1995

5. Effects on the brain and behaviour

5.1 Acute effects on central nervous system functions and behaviour

The acute effects of cannabis use have been recognized for many years, and features such as mild euphoria, relaxation, increased sociability, heightened sensory perception and increased appetite have been described in earlier reports. The acute effects of higher doses, including perceptual changes, depersonalization and panic have also been well described previously (ARF/WHO, 1981). Research conducted since the last WHO report has focused mainly on quantifiable effects such as those on memory, psychomotor performance and appetite; however, some work has also been done on the acute psychotropic effects of cannabis. A recent report done by Mathew et al. (1993), showed that cannabis smoking was associated with significant depersonalization that was maximal 30 minutes after smoking. Other behavioural changes associated with cannabis intoxication included loss of time sense, sensation of 'high', anxiety, tension and confusion (Mathew et al., 1993).

Recent studies have confirmed and extended earlier findings that cannabis can affect memory in various ways. Free recall of previously learned items is often impaired when cannabis is present both during the learning and the recall; the major impairment is often reflected by intrusions and novel items. In a study done by Block et al. (1992), the acute effects on human cognition of cannabis were assessed. It was found that cannabis impaired all capabilities of learning, including associative processes, and psychomotor performance. The only areas that were not affected were those of abstraction and vocabulary. It was also found that cannabis use altered associative processes, thereby making uncommon associations more pervasive (Block et al., 1992). Recall of prose material is generally impaired by cannabis, but its effects upon recall of series of numbers (the digit-span test), recognition, and paired-associates tasks (arbitrary word pairings) have been inconsistent. Generally, material learned in the absence of cannabis can be recalled even if cannabis is present in the blood. Although the acute effects of cannabis on memory appear to be modest, the possibility must be considered that chronic use by an adolescent might result in a cumulative developmental impairment.

A substantial number of recent studies have confirmed that cannabis use consistently increases the consumption of food, especially of high carbohydrate foods. In contrast, cannabis has not shown consistent effects upon 'the users' subjective reports of appetite. The reason for this apparent dissociation between appetite and food consumption is not known (Mattes et al., 1994).

Several studies have shown that cannabis appears to increase the perceived rate of the passage of time. Consistent with earlier observations, numerous studies in the past ten years have confirmed that cannabis impairs psychomotor performance in a wide variety of tasks, such as handwriting, tests of motor coordination, divided-attention, digit-symbol substitution, and operant tasks of various types (Solowij et al., 1991). The consistency of results is probably attributable to improved experimental technique, as reflected by greater attention to the importance of task complexity, standardization of THC administration, studies of dose-effect relationships, and of sharper definition of acute versus residual effects.

A number of studies have examined the acute effects of alcohol and cannabis on various performance tasks, but the results have been quite varied. In almost all cases the combination had a greater detrimental effect than that of either drug alone, but in some cases the effects were fully additive, in other cases incompletely additive, and in a few instances apparently antagonistic. This is an important area for further research.

Cannabis has been shown in a variety of ways to function reliably as a reinforcing agent, and the degree of reinforcement is proportional to the THC content (Gardner & Lewinson, 1991).

Relatively few studies have examined the effects of cannabis on a variety of social behaviours, including verbal and aggressive behaviours.  A somewhat inconsistent pattern of changes has been reported following cannabis use, and the behavioural effects of cannabis can be influenced by the social context of use.

Finally, residual effects of cannabis administration and effects of cannabis and other licit and illicit drugs are poorly defined and require more research. Cannabis is often used concurrently with other psychoactive substances. The resultant effects on human behaviour have received less attention and yet the impact polysubstance use has on health and safety may be substantial.