Marilyn,
I agree with Matt on the decision to try heat therapy before antibiotics. This usually works well if the RI is in the beginning stages. However, if it does not show some improvement in a week or so, and you are certain about the diagnosis, I would think about antibiotic therapy. From your description it does not sound like advanced RI, as it is still feeding, and it could probably be treated effectively with ceftazidime, ciprofloxacin, or Baytril. However, I would discourage the use of Baytril. While both ciprofloxacin and Baytril are fluoroquinolone antibiotics, Cipro has a broad spectrum of activity against both gram positive and gram negative bacteria and has proven to be more effective than Baytril. Cipro is a tablet and this works great if the snake is still feeding like yours is. Just partition off the tablet in sections based on the needed dosage per weight of the snake. The safety factor of Cipro is large and an estimate like this is not a problem. Just insert the sectioned piece into the smallest food item the snake will eat. If the RI becomes advanced, it is unlikely that any of these antibiotics will be effective and your best bet then is an aminoglycoside antibiotic such as amikacin or tobramycin. Although they are not as safe as the first group, they can be used safely with the proper dosage. Contrary to popular belief, it is not necessary to manually hydrate the snake when using aminoglycosides, but you must always have fresh water readily available at all times. Preferably in a low sided bowl. Just keep the humidity fairly high, ( 70 to 80 % ), but not overly high, and temps in the upper 80’s to 90 with no cool side. I hope this will give you a little more information to work with. Good luck.
Kelly