Hi. Thanks for asking in Healthcare Magic. The antibiotics like Ampilcillin, Amoxicillin, Tetracyclines, Clarithromycin and Co-trimoxazole interfere with the function of plan B by suppressing the intestinal microflora and decreasing the duration of action of Levonorgestrel present in plan B.Since you have already taken Plan B and the above said antibiotics will reduce the efficacy of plan B you can take some other antibiotic like Cephalosporins or Azithromycin. Although Azithromycin belongs to the same group of Clarithromycin it does not have any interaction with Levonorgestrel (progesterone) and only interferes with estrogen activity. Since plan B does not contain estrogen, it is safer to take Azithromycin. Also Azithromycin is very effective in throat infection. Your treating Dr will know about these drugs. I hope this is useful to you.