I use the thesaurus constantly, on the basis I have a wide vocabulary but if I had to wait for my memory to provide me with the precise word for which I'm cudgelling my brains, I'd still be on chapter 1. However, I rarely use a word with which I'm unfamiliar and never without double-checking its meaning and instances of how it's been used.
I'm a little concerned, though, that you might be doing the opposite. Using a thesaurus and a good dictionary to improve your own vocabulary is an excellent idea. (As to which, this thread might interest you
http://www.sffchronicles.co.uk/forum/50858-eloquent-exotic-esoteric-please-provide-definitions.html) Sprinkling words into a story which you don't know and have never met before because they sound "better" can be a terrible mistake -- I've seen many examples of people trying to write in a more high-falutin' style, thinking it's literary, and they come a cropper because the words they've used are wrong in context.
As for how many words you can use which are unfamiliar, it's difficult to say as different readers will have different tolerances. I've seen pieces here in Critiques where some readers have pulled out words as being difficult when to me they were perfectly ordinary (thinking about it, that includes pieces of my own...
). I'm a great believer in stretching people and dropping a few unfamiliar words in, though I try and make it clear in context what they mean, but I have to confess I get annoyed when confronted by something where a lot of words aren't known to me -- I then instantly decide the arcane and esoteric are being used to show off, rather than because the words are absolutely the right ones for the story!
As for panacea and zenith, they're to me wholly unexceptional, and I wouldn't turn a hair
if the words are being used correctly
and by characters who would indeed know and use them.