Prevacid is a Proton-Pump inhibitor (PPI), and long term and/or high dose use of PPIs has been shown to cause some side effect problems.
I take 20 mg omeprazole daily (a modest over-the-counter dose of another PPI), which controls my heartburn pretty well. But occasionally the symptoms breakthrough the PPI. On those days instead of doubling the PPI dose, I take one OTC dose of ranitidine - another acid-reducing drug that is not a PPI. The combo seems to work to knock down the problem on the bad days.