@anamur-- You've touched on a great point. Omeprazole does reduce the absorption of vitamins, especially vitamin B12. There might be a connection between vitamin deficiencies and weight gain during proton pump inhibitor use, but I'm not sure if it applies to everyone.
I've gained twenty five pounds since I started my omeprazole treatment one year ago. But I am not over-eating, so I don't think that my weight gain is due to an increase in appetite. My doctor also told me that omeprazole is the cause and that I will probably go back to my normal weight once I stop the medication.