Does Prilosec prescription have any reports of serious side effects. Also does this pill cause weight gain.?

Yes it does. As you can imagine any medication can cause side effects. I looked up in a pharmacopeia and Prilosec can cause liver disease, severe skin blistering that requires emergency treatment. Abnormal absorption of certain electrolytes like phosphate. It can cause osteopenia or osteoporosis in certain people. I am not at all familiar with weight gain being one of the side effects.