According to the PDR Prevacid information:
”Additional adverse experiences occurring in <1% of patients or subjects in domestic trials are shown below……
Metabolic and Nutritional Disorders – gout, dehydration, hyperglycemia/hypoglycemia, peripheral edema, weight loss/gain. ….”
This means that weight gain is very rare but possible side effect. Possibly it may be a “water” weight.
More than likely a spurious association, though.
I experienced bloating while taking Prevacid and reported that to my doctor for example. I eventually found out the Prevacid had nothing to do with it. The same types of things can happen in these clinical trials.