The combination of alcohol and marijuana can lead to tachycardia (a resting heart rate higher than 100 beats per minute); raised blood pressure; and increased impairment of cognitive skills, motor skills, and driving performance as compared with the use of either alcohol or marijuana alone [sources: American Heart Association, NIDA].
Of course, you shouldn't be driving if you've used even one of these substances on its own, but when you use them together, the risk is even greater. A study by researchers at Maastricht University in the Netherlands found that both low doses of alcohol and low doses of THC (the active ingredient in marijuana) moderately impaired driving performance when administered alone. When the same low doses of alcohol and THC were combined, the effect on driving performance was severe. Although the alcohol provided to subjects was only enough to produce a blood alcohol content (BAC) of 0.04, the subjects' driving performance when THC was administered in conjunction with the alcohol was consistent with BAC levels between 0.09 and 0.14 [source: Ramaekers et al.].
Even if you're not driving, the combination of alcohol and marijuana can cause nausea, vomiting, dizziness, panic attacks, anxiety or paranoia, particularly if you drink alcohol before using marijuana, rather than the other way around [source: NCPIC].