Many times products (as different salts) are produced by companies for
'marketing' purposes, e.g., Pfizer/Roerig (Roerig was a subsidiary
ompany)  --  Atarax (antihistamine) as the hydrochloride, Vistaryl
(tranquilizer) as the pamoate.  Same drug.

Other reasons might be for 'getting-around' other company's patents (can't
think of an example at the moment).

An excerpt from a website
(http://www.rxlist.com/cgi/generic3/propoxnap.htm ) states:  Propoxyphene
napsylate differs from propoxyphene hydrochloride in that it allows more
stable liquid dosage forms and tablet formulations. Because of differences
in molecular weight, a dose of 100 mg (176.8 µmol) of propoxyphene napsylate
is required to supply an amount of propoxyphene equivalent to that present
in 65 mg (172.9 µmol) of propoxyphene hydrochloride.

Again the same drug.  Therapeutically, I believe there is no significant
difference.

HankG