"Head and Shoulders" and "Body Zone" both sell anti-dandruff shampoo
with the active ingredient 1% zinc pyrithione. Body Zone is
significantly cheaper which is why I bought it--assuming everything else
was equal. Yet the Body Zone doesn't work as well. I expected it to work
just as well as Head and Shoulders.

My hypothesis is that there some deterioration in the effectiveness of
the chemical over some period of time--that Body Zone gets its zinc
pyrithione at a much cheaper price because it is an older batch than the
zinc pyrithione that Head and Shoulders uses, and is therefore cheaper,
but less effective.

My questions to this group are these: a) is there merit to the idea? b)
are there other factors that might mitigate the issue?