In general, the use of marijuana can alter the perceptual functions although does not cause changes in sensory functions . The alteration of perceptual ability and difficulty concentrating or maintaining attention to their use of cannabinoids have been confirmed by simulation studies and real driving test. In these studies it was also found that subjects under the influence of these substances are aware of their state of alteration and tend to compensate by driving at low speeds, following distances to the vehicles that precede them, avoiding overtaking. Cannabis is the main illegal substance found in injuries from motor vehicle accidents, and frequently it is found associated in combination with alcohol or other drugs of abuse.
The drivers tested positive for marijuana with THC concentrations in the blood of more than 5 ng / mL were responsible for incidents at a rate significantly higher than the drivers test negative. Cannabis is eliminated very slowly by the body, in a study of regular smokers of cannabis were found measurable concentrations of THC even after a week of abstinence from smoking, and some of the subjects still had problems in the tests of driving ability. In a study of habitual heavy smokers, the presence of THC in the blood for an extended period thus seems to lead to cognitive deficits even after a long period of abstinence from smoking.
The effects are divided into acute and chronic. The acute effects are associated with the recruitment of a single dose of drug, while those chronic derive from the use of a substance for a long period of time. These effects are amplified in the case of association of cannabis with alcohol or other psychoactive substances.
Effects on driving skills :
Reduced perception to light stimuli peripheral
Reduced oculomotor control
Difficulty with coordination
Poor speed control, braking and acceleration errors
Minor judgement, risky overtaking manoeuvres
Impairment of attention, especially to perform multiple actions simultaneously
Reduction of the short-term memory
The acute effects
The acute effects of cannabis affect psychomotor functions needed to control and reduce intensely motor control, psychomotor speed, executive functions, the short-term memory and working memory (reaction time and accuracy). In general, the effect of cannabis - the phase "high" - lasts up to 2 hours after ingestion, although many studies highlight the continuing negative effects on cognitive and motor functions up to 10 hours after use.
The chronic use of cannabis may lead to a deterioration of memory processes, attention, manual dexterity, executive function and psychomotor speed. These effects may persist beyond the period of intoxication and worsen depending on the duration and frequency of use of the substance. These deficits are for the most part reversible through abstinence prolonged in time. However, many of these can be permanent.