What happens when cannabis and alcohol are combined?

Cannabis-alcohol combination ("crossfading") produces synergistic impairment that is substantially greater than either substance alone. Pharmacokinetic interaction: alcohol increases THC absorption — a 2001 study found alcohol increased peak THC plasma levels by 114% compared to cannabis alone. This explains why the combination produces more intense intoxication than expected from either substance. Pharmacodynamic interaction: both substances impair the same cognitive and motor functions (reaction time, divided attention, tracking ability) through different mechanisms — their impairment effects are additive to synergistic. Driving impairment: the combination produces greater driving impairment than either alone; a 2015 NHTSA study found the combination increased crash risk more than either substance individually. Nausea: paradoxically, cannabis can reduce alcohol-induced nausea (antiemetic effect), potentially enabling higher alcohol consumption. Blackout risk: the combination may increase blackout risk. Cardiovascular: both cause vasodilation and can cause orthostatic hypotension — the combination increases syncope risk. There are no known fatal overdoses from cannabis-alcohol combination, but the impairment risk is significant.