Histamine is a chemical produced throughout the body, and it has a number of effects. One is to stimulate the acid producing cells of the stomach (parietal cells) to produce more acid. These cells receive the signal when histamine attaches to what are called Histamine-2 receptors. These drugs block that process, resulting in a significant reduction in acid production. These medications were only available with a prescription up until 2003. The OTC version is the same drug as the one that requires a prescription, but in a lesser dosage.
This group of drugs has become the most popular treatment for GERD and other acid related problems, largely because they are incredibly effective at reducing stomach acid. They include familar names like Prilosec OTC®, Prevacid®, Zegerid®, Dexilant® and the most popular of all, that little "purple pill" Nexium®. These medicines work by stopping the production of acid by the stomach. They are often extremely effective at reducing or even eliminating heartburn symptoms, but to do so they must be taken on a regular basis.