PPIs, which include heartburn drugs such as Prilosec, Nexium and Prevacid, are one of the best-selling classes of drugs in the world.

Brand names for such medications -- one of the top-selling drug classes in the United States -- include Prilosec, Prevacid and Nexium.

The research raises more questions about commonly used drugs such as Nexium, Prilosec and Prevacid, which had long been considered effective with few side effects.