The reason why famotidine loses its effect after repeated use is due to a phenomenon known as tachyphylaxis. This is a class effect which means drugs similar to famotidine, otherwise known as the H2-blockers, share the same characteristic. H2-blockers work by blocking the effects of histamine on the cell receptors, with the end result of reducing stomach acid production.
The real reason for tachyphylaxis to occur remains largely unclear although a widely postulated theory is the up-regulation of other receptors at the surface of cells such that more pathways now control acid production besides histamine. As such, blocking the effect of histamine on the cell surface alone becomes insufficient for adequate acid control.
One way to minimise this effect besides changing the class of medicine is to have "drug holidays" and use H2-blockers on and off, rather than regularly. If this fails, other options such as proton pump inhibitors (PPI), which does not have this problem, may be your answer.
