Yep...penicillin works great for the part you can see....but what you can't see or feel is the incredible burn when it goes in. By using a buffering agent like Lactated Ringers it cuts down on some of the burn. Ask any old school doctor about the burn with penicillin injections. Telling my age again...but Iremember some penicillin injections...not comfy at all. The dosing of procaine is usually quite small so I mix it in with about 10mL of LRS. And honestly, if a rabbit has an infection that requires that type of antibiotic, it probably has an elevated body temp so the LRS would probably be helpful as well. I use a different approach to injecting meds and fluids. I use a butterfly catheter withluer lockon a large syringeas it gives some "wiggle" room...literally over a hard fixed syringe. By buffering the drug, it also decrease the chance of developing the skin abscesses especially if the penicillin is injected near the same spot all the time. Another drug we use in sick animals, B Comp, also stings quite a bit going in as do several others.
Randy