I believe there isn't currently any mechanism for the U.S. FDA to restrict number of pills sold of OTC medication, only the contents of each individual pill and the dosage instructions. The assumption is that OTC means purchase itself is unregulated, so you can order a truck-load if you really want to stock up.

The only cases I'm aware of where purchase numbers are limited for OTC drugs is when separate laws targeting drug precursors impact them, which is why you can't buy large numbers of pseudoephedrine tablets at once (they can be used to manufacture meth).

The FDA does seem to be reducing the maximum dosage of each tablet, though: from 2014 the maximum will be 325mg, so the 500mg tablets linked above will be phased out. But that change seems aimed at reducing accidental liver damage from excessive "normal" use, rather than aimed at reducing suicides from deliberate overdoses.