Answer: This has nothing to do with how the tablets are made. The answer above is correct - the citrate and the hydrochloride forms of diphenhydramine are just different salts. Different salts have different properties, such as solubility, and may improve the bio-availability of the drug in question. They also contribute to the stability of the drug (i.e. the ability to maintain efficacy over storage time). There are many things that the difference in salts can affect, but in marketed products such as Excedrin PM and Tylenol PM (or Simply Sleep) there is virtually no difference between the 2 diphenhydramine forms.