There have certainly been relevant studies. (e.g. "Long-term safety and clinical acceptability of venlafaxine and imipramine in outpatients with major depression", RK Shrivastava et al, Journal of Clinical Psychopharmacology, Oct 1994). However accessing such studies, and making sense of the results, isn't easy.
It might be easier to consult the official information for the drug (as approved by the US Food and Drug Administration)
http://www.drugs.com/pro/venlafaxine.html
and/or Wikipedia:
http://en.wikipedia.org/wiki/Venlafaxine
Chris



