I take 7.5 mg of Cortef twice a day. I've been taking it for several months and have not noticed any weight gain. But I didn't have much of a weight problem before I started taking it. Isnt' it excess cortisol that cause weight gain? If you are low perhaps bringing your level up to somewhat normal won't cause you to gain weight.
I've been on a program that includes the Cortef, Adrenal supplements, Pituitary, Panothenic acid, DHEA and vitamins and EFA's. I started feeling better with more energy and some improvement in symptoms. Then the stress level increased dramatically in my life and bam - almost back to square one. But I'm glad I'm trying this protocol.
If you start suddenly gaining more weight you can stop the Cortef. It's probably worth a try.