Effects of drugs on driving
CNS stimulants
In contrast to the sedation-inducing drugs discussed above, stimulants generally increase neuronal activity which, in moderation, may not be entirely detrimental. Caffeine can revive the drowsy driver, and patients with attention deficit disorder (ADD) or narcolepsy can benefit from appropriate doses of methylphenidate or amfetamine. However, stimulant drugs can upset the delicate neural homeostasis in a healthy individual. Stimulants, principally cocaine and the amfetamines, are used recreationally for their excitatory, euphorigenic properties. In this context they produce intense overwhelming and distracting effects. After acute administration, users report feeling elated, powerful, having superior intellect and insight, and intense sexual arousal and stimulation. Time may appear to pass more quickly, speech becomes faster and less coherent, and users can become impatient and agitated, sometimes to the point of violence. These perceptual changes are accompanied by increases in pulse and blood pressure, pupillary dilation, sweating and psychomotor restlessness, manifested as pacing, fidgeting and scratching. Simple reaction time may be improved under the influence of stimulants, but this is only one component of driving; in fact, complex reaction time, which demands impulse control, intelligent decision-making and appropriate response, may be affected adversely. The intensity of these effects depends on the dose, the route of admin istration, and to some extent the user’s experience with, and tolerance to, the effects of the drug. Inevitably, these effects, when combined, are detrimental to complex task performance and make drivers less attentive, while the psychomotor excitation demands greater focus on muscle control and vehicle operation. These opposing effects result in poorer driving. As with any other drugs, the effects are likely to become more apparent when demands are high, such as driving in bad weather, in heavy traffic, in an unfamiliar environment or in a defective vehicle. In addition to these acute effects, humans display both acute and chronic tolerance to the effects of stimulants. With cocaine and the amfetamines, the initial excitatory, euphoric phase can be followed by a withdrawal phase, the intensity of which depends on the duration and intensity of drug use. During the withdrawal phase, subjects are typically fatigued, lack energy, and can be irritable, depressed and restless. Sometimes delusions and pseudohallucina tions can occur, and the subject can become psychotic. Often this is called the ‘crash’ phase, during which the subjects sleep a fitful restless sleep, sometimes for days in the case of metham fetamine withdrawal. Drivers have been reported as being lethargic, sleepy, with very poor lane discipline (weaving), and frequently drive at high speed, drive off the road or drive into oncoming traffic.
Blood cocaine and amfetamine drug concentrations must be interpreted with caution, since a single blood drug measurement does not predict what phase of intoxication the subjects may be experiencing, and consequently what pattern of effects are likely to predominate. Nonetheless, blood drug concentrations can show whether drug use is consistent with recreational doses in the case of cocaine, and may help distinguish between legitimate dosing of amfetamines (e.g. in the treatment of narcolepsy, ADD or eating disorders) and recreational use. The doses of stimulant drugs required to produce the sought-after euphoric effects are accompanied inevitably by deterioration in driving performance.
