Hi. I too struggled to find specific information regarding drinking whilst on Effexor. The enclosed leaflet read "Avoid alcohol" without explanation.
From what I understand, alcohol may increase some of the potential side effects of this drug such as nausea, drowsiness, abnormal dreams, clumsiness, disorientation, rapid heart beat etc.
My boyfriend is the manager of a nightclub. I occasionally drank a small amount of alcohol whilst on a low dose of Effexor (37.5mg daily) without problems. Although it should be noted that I wasn't experiencing any side effects anyway, except for slight constipation.