How well are commonly used antibacterial agents actually absorbed from the GI tract?

Recent trend toward earlier transition from parenteral to oral antibacterials in many conditions, such as pneumonia and urinary tract infections, makes it imperative for clinicians to be familiar with the bioavailability of commonly used agents (1,2).

Agent                                                                                Absorption (%)


Amoxacillin                                                                       74-92

Amoxacillin/clavulanate (Augmentin®)                  60

Azithromycin (Zithromax®)                                         37

Cefpodoxime (Vantin®)                                                 30-50

Cefuroxime axetil (Ceftin®)                                         30-52

Cephalexin (Keflex®)                                                     90-100

Clindamycin (Cleocin®)                                                  90

Ciprofloxacin (Cipro) ®                                                   70

Doxycycline                                                                     >90 with food

Levofloxacin (Levaquin®)                                             >90

Metronidazole (Flagyl®)                                                 80

Moxifloxacin (Avelox®)                                                 88

Trimethoprim-sulfamethoxazole (Bactrim®)        70-90


It’s worth noting that the aborption of fluoroquinolones (e.g. ciprofloxacin, levofloxacin, and moxifloxacin) is reduced by concurrent administration with magnesium-aluminum antacids and sucralfate, and in decreasing order, iron, calcium and zinc (3). Spacing the doses of these agents from potential interacting agents is advised.



  1. Zhanel GG, Walkty A, Vercagine L, et al. The new fluoroquinolones: a critical review. Can J Infect Dis 1999;3:207-38.
  2. MacGregor RR, Graziani AL. Oral administration of antibiotics: a rational alternative to the parenteral route. Clin Infec Dis 1997;24:457-67.
  3. Lomaestro BM, Bailie GR. Absorption interactions with fluoroquinolone:1995 update. Drug Safety 1995;12:314-33.


How well are commonly used antibacterial agents actually absorbed from the GI tract?

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