There are a number of possible side effects with Effexor XR, and weight gain is a rare side effect that has been documented in at least 1 percent of patients. There are some things that you can do if you experience weight gain while taking Effexor XR, such as eating a heart-healthy diet, exercising regularly, and limiting your alcohol intake. Your healthcare provider may be able to give you some other suggestions for dealing with any weight gain that may occur.