The detrimental effects of drinking alcohol and smoking cannabismay be additive on some aspects of driving performance. However, there is some evidence that regular cannabis use per se does not potentiate effects of alcohol. Smoking cannabis may alter thebioavailability of alcohol.
Simultaneous use of alcohol and oral :8.7pt; font-weight:normal; color:#000000″>Δ(See reference number 9)-tetrahydrocannabinol (THC, major active ingredient of cannabis) reduced performance of psychomotor tests, suggesting that those who use both drugs together should expect deleterious effects to be additive (See reference number 1). In a further placebo-controlled study,subjects smoked cannabis containing 100 or 200 micrograms/kg of :8.7pt; font-weight:normal; color:#000000″>Δ(See reference number 9)-tetrahydrocannabinol and drank alcohol (to achieve an initial blood level of 70 mg%,with further drinks taken to maintain levels at 40 mg%) 30 minutes before driving. They found that cannabis,even in low to moderate doses, negatively affected driving performance in real traffic situations. Further, effect of combining moderate doses of both alcohol and cannabis resulted in dramatic performance impairment as great as that observed with blood-alcohol levels of 140 mg% alone (See reference number 2,3).
One study in 14 regular cannabis users (long-term daily use) and 14 infrequent cannabis users found that regular use reduced disruptive effects of alcohol on some psychomotor skills relevant to driving, whereas infrequent use did not have this effect. In this study, neither group had smoked any cannabis in 12 hrs before alcohol test (See reference number 4). Another study found that moderate doses of alcohol and cannabis, consumed either alone or in combination, did not produce significant behavioural or subjective impairment following day (See reference number 5).
A study in 12 healthy subjects who regularly used both cannabis and alcohol found that alcohol 0.5 g/kg significantly increased break latency without affecting body sway,whereas cannabis given as a cigarette containing tetrahydrocannabinol 3.33%,increased body sway but did not affect brake latency. There were no significant additive effects on brake latency, body sway, or mood when two drugs were used together (See reference number 6). A population-based study of 2,777 drivers involved in fatal road crashes, who drank alcohol and/or used cannabis, found that although both cannabis and alcohol increased risk of being responsible for a fatal crash, no statistically significant interaction was observed between two drugs (See reference number 7).
Fifteen healthy subjects given alcohol 0.7 g/kg developed peak plasma alcohol levels of 78.25 mg% at 50 minutes, but if they smoked a cannabis cigarette 30 minutes after drink, their peak plasma alcohol levels were only 54.8 mg% and they occurred 55 minutes later. In addition, their subjective experience of drugs decreased when used together (See reference number 8). However,another study found that smoking cannabis 10 minutes before alcohol consumption did not affect blood-alcohol levels (See reference number 5). A further study found that blood-alcohol levels were not affected by :8.7pt; font-weight:normal; color:#000000″>Δ(See reference number 9)-tetrahydrocannabinol given orally one hour before alcohol (See reference number 1).
Several studies have found that cannabis and alcohol produce additive detrimental effects on driving performance,but other studies have failed to show any potentiation. This is probably due to variety of simulated driving tests used and possibly time lag between administration of alcohol and cannabis; behavioural impairment after cannabis has been reported to peak within 30 minutes of smoking (See reference number 5). Nevertheless, both drugs have been shown to affect some aspects of driving performance and increase risk of fatal car accidents. Concurrent use of cannabis and alcohol before driving should therefore be avoided.
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