Will the weight/appetite gain of marinol last?

I have a friend that has a deep underlining problem of maintaing a healthy weight due to lack/loss of appetite due to a childhood illness that affected that sensory. She has tried many medications during the years to try to gain weight. Or to increase appetite. At one point, a medication called zyprexa helped her gain ten pounds, and after eight months or so, it started to dwindle and she eventually lost the weight she gained and even then some. The doctor has prescribed marinol to see if it works... question she is concerned with is: will it help her eat more and if so will it last this time? Thanks in advance!