Cocaethylene is a psychoactive compound formed in the body when both cocaine and alcohol have been administered together. It has similar pharmacologic properties to cocaine, blocking the re-uptake of dopamine in the brain. However it has a much less potent serotonergic inhibitory mechanism, so the effects are much more euphoric and rewarding than cocaine.
Cocaethylene is more toxic to the cardiovascular system than either cocaine or alcohol alone, making the combination of cocaine and alcohol much more potentially lethal.
Sources:
http://cocaine.org/cocaethylene.htm