Effexor side effects seem to defeat the intended purpose of the drug. It is supposed to be used in the treatment of depression. However, it is known to increase the suicidal thoughts and actions of some patients. In addition, it can cause serotonin syndrome in people who also take medication for migraine headaches. If that is not enough, Effexor can cause seizures, bleeding problems, irregular heartbeat, and more. If you feel that you or a loved one is suffering from Effexor side effects, consult your doctor immediately. Do not stop the drug cold turkey, as this can also cause adverse health problems.