It depends on the medicine. I have given injectable ivermectin and LA200 (oxytetracycline) orally with no problems. Tylan, I think, is okay by mouth. But, you'll need to administer it directly to the rabbit, not in the water; because they'll either not drink
enough to benefit (which results in resistance to the med) or they'll totally refuse to drink at all & become dehydrated. The only exception to that is Corid to treat coccidia, which is one capful per gallon of water (and they may refuse to drink it).
NEVER give penicillin (or any "cillin") to a rabbit by mouth. They cannot tolerate it orally and will quickly die (I know from
experience).
Some additional information:
Antibiotics will NOT cure pasturella (white nasal discharge/sneezing/goopy eyes), it will only mask the symptoms, while the animal continues to infect others.
Vent disease (rabbit syphilis) is only cured by Pen-G injections (the long-acting penicillin). Dosage is 1/10 CC / lb, once a week for 3 weeks. Topical penicillin (like Go-Dry) will not cure vent disease, as it doesn't allow the med to get in the blood stream.
Penicillin, Oxytetracycline or Tylan can be used for abcesses, head tilt or mastitis at 1/10 CC per lb, every other day. They may or may not be effective, depending on the
root cause of the infection.
Honestly, I rarely treat anything anymore, except for vent disease. For anything respiratory or digestive-related I prefer to just cull, as I only want the genetics for the strongest immune systems getting passed down.
But, anyway, I hope this helps!