Hi Gail: Lexapro is an SSRI and in the same class of medication as Prozac and Zoloft. Pristiq has only been on the market since January 2007, so it is hard to predict how the public at large is going to react. But, it's an SSNRI like Cymbalta and Effexor. Weight changes are common in both classes of medications. You may want to ask your doctor if there are other medications that do not include weight gain for your daughter's OCD. This is a difficult problem to treat. If you have a medication that is working, it might be worthwhile to investigate ways to control the side effects.   Best of Luck, Dr. Diana Walcutt   *Dr. Walcutt is a Psychologist. Dr. Walcutt's answers are not intended to diagnose or treat any medical or mental disorder. Any information given in a post about medication is for educational purposes only, and primarily to aid you in having an informed discussion with your own Psychiatrist/Physician.