Never. Or at least several months away from effexor. There is a post here where a member took an antidepressant and DXM and got serotonin syndrome. The member actually died. Let me see if I can find that post and link it here. Someone will have a better answer for you.
Just be careful and you are very wise for being careful. I wish more people would ask questions. Your health is important to you and your family AND us! Let me see if I can find that thread. It shook up a lot of BL members.