Due to the complicated balance of bacteria in a rabbits gut, you need to be very careful treating bad bacteria with antibiotics that might upset the good bacteria. Some antibiotics used with other animals are fatal to rabbits as a result, but even the safer ones come with risks.
Our Patty is a good example of this - after a few days of antibiotics a few weeks ago to treat one issue, her digestive system has seemed like it is in meltdown since. She suffered bad diarrhea and crashing weight loss, her condition went from good to almost terminal in a very short space of time. As a result, while fighting to keep her going we have needed to be very careful about what goes in that mouf of hers to make sure her condition doesn't further deteriorate. She is on a carefully controlled and monitored regime trying to bring her system back into balance, which so far has included syringe foods/supplements, wild plants and dried herbs - crucially (much to Benson's annoyance, since this generally means he gets lumbered with the same restrictions by association), our vet has warned against giving Patty vegetables. The one exception we are allowed to give her is an occasional and small amount of fresh herbs.
With all the attention, her strict diet and daily bottom washes, it is a difficult and stressful time for Patty (and Caroline of course, being the one who has to clean that messy tail!), we are all really hoping she starts to improve soon and gets through this. In the meantime, I can say this for sure - she really does love those occasional parsley treats!