According to a scientific study conducted by two German Universities, THC can increase the toxic effects of Alcohol and other drugs.
Researchers at the Neuroscience Research Center of the Humboldt University in Berlin and the Department of Pediatric Neurology, University of Technology Dresden, Germany conducted tests on rats between one and fourteen days old:
The results showed that THC and its synthetic form did not cause neurodegeneration when administered alone but did cause cell death when given with lower than toxic amounts of ethanol. This combined effect increased according to the dose of THC that was administered and was strongest when the rats were 7 days old. THC also enhanced the neurotoxic effect of phenobarbital and MK-801 (ethanol combines the mechanism of action used by these two drugs). In addition, marijuana activates CB1 receptor levels, which causes the psychomotor, memory, cognition and pain perception changes seen with this substance in adult humans and animals. In the current study, THC combined with ethanol increased these levels...
So the lesson here is, don't let your newborns mix their drugs folks.
Seriously though, this study is probably a good reason to stop mixing your drugs. In my younger Pot smoking days, I learnt the same lesson. I could be guaranteed to throw up if I tried having a scoob after a gutful of beer. Strangely though, if I smoked first, I didn't get the same effect. But that's probably because I drank far less if I was stoned first.