Cipro/Levaquin: Interact with anti-rejection meds?

I asked my transplant doc if I could have a scrip for either Cipro or Levaquin to fight my sinus infection. I'm allergic to penicillin, I've had a rash from Ceftin, so I can't take those anymore. My transplant doc refused, saying that Cipro and Levaquin interact badly with anti-rejection meds. (So he gave me a scrip for Zithromax instead, which so far hasn't knocked out the infection.)

This was a real surprise to me. Folks on this very forum have said that they take Cipro as part of their anti-rejection cocktail, so I was surprised to hear that it cannot be taken concurrently with anti-rejection meds.

So what's the story here? Which anti-rejection meds do Levaquin and Cipro interact badly with?