jheem said elsewhere, "It all depends on your definition of a dictionary. For some it is a prophylaxic against bad grammar ..."

I was aware of the words prophylactic and prophylaxis, but not prophylaxic. But never doubt jheem! And indeed, although the last is not in one-look, it's found in use.

Is there any difference in usage between prophylactic and prophylaxic?

Yes, as you saw in the link, wordnerd, "prophylaxic" is used medically, though "prophylactic" is also used medically.

I don't think there is a difference between the words, but I could be wrong.

Of course, we all know the noun use of a "prophylactic."

-xic is a re-formation in English: the Greek endings were always -tic- but -sis/xis, with assibilation having taken place in the Attic-Ionic dialect.

Learn to doubt, expecially if I'm jet-lagged with the flu. I started to write prophylaxis and then changed words midstream ending up with what I ended up with. I should've written French letter.