Sam,
I'll preface by saying I am not much of a fan of search
functions (or "googling" as it is now known),but here are some
thoughts:
If you have a few key items, like "co-pay," that can confuse
the user, you can add those to your content and hopefully the
search will put these at the top. For example, you could write
"copay (sometimes written as 'co-pay')" in the first instance where
it is introduced.
It is hard to foresee what the user might search for, but
this is the same thought process you would use in creating an
index. Instead of index items, you would be adding them in the
content.
Otherwise, you would need to instruct your users on how to
use the search function - when to use quotes, AND, etc.
John