welcome to the Pain Management Forum. I am glad you found us. You are a wonderful daughter to be so concerned for your mother. She is blessed to have you.
I have two thoughts. I am wondering if rather than the morphine that is causing her weight loss, could it be the pain and the surgery itself. Pain can cause loss of appetite (or increased appetite) and could be contributing to the picture. Was the surgery successful? Does she have more pain as as much pain as she did prior to surgery.
You don't say how old you Mom is and that could be another huge factor. Is there something else going on with your Mom that could contribute to or be the cause of the weight loss?
Can you encourage her to eat more or better. There are many supplements out there. What is her physicians response to her weight loss? Morphine itself, does not cause weight loss. It is more likely that the other medicines she may be taking in conjunction with the morphine may be the culprit.
I would discuss your concerns with her prescribing physician. With the new HIPPA laws her physician will not be able to talk with you without her permission. However you may offer him/her information on your mother's condition. You may want to make an appointment with her physician and than accompany her to address your concerns in her presence.
Good luck to you. I am caring for an elderly mother (and father) that have multiple physical issues. I know the concern that goes with an ill parent. Please feel free to PM me and keep us posted. I will look forward to your next update. Take care, Tuck