What’s the difference between oral (PO) and IV medications? When do you use PO vs. IV antimicrobials? When are they interchangeable? These are the questions we’ll address in this post.

Bioavailability is an important concept to understand when considering IV to PO interchange.

Bioavailability = the measure of the amount of an orally administered medication that reaches the bloodstream.

Antimicrobials with >90% bioavailability are the antimicrobials we can target for
IV to PO interchange.

Antimicrobials where bioavailability >90%:
(therefore, can be switched to PO)

• Metronidazole
• Fluoroquinolones (levofloxacin, moxifloxacin, *ciprofloxacin has ~70% bioavailability but still has enough to achieve adequate levels in the bloodstream)
• Trimethoprim-Sulfamethoxazole
• Tetracyclines (doxycycline, minocycline)
• Linezolid
• Rifampin
• Fluconazole/Voriconazole
• Clindamycin
• Azithromycin (only ~40% bioavailable, but the concentration achieved by
  oral ingestion is just as effective as IV for treatment)

IV medication = medication given intravenously
– medication takes effect immediately after the infusion
– administers a bolus of the medication quickly (within 5 minutes)
– requires an IV line
– bypass first pass metabolism in the liver

PO medication = medication administered per oral route
– medication takes effect in ~30 minutes to 6 hours
– requires ability to swallow, absorb the medication, and also undergoes
first pass metabolism prior to reaching the circulatory system

Why is PO preferable to IV?

• Cheaper³
• Does not require IV access
• Easier and faster to administer
• No IV complications (i.e. phlebitis, thrombosis, bloodstream infection)
• Avoidance of a long-term catheter such as a PICC line
• Less unnecessary fluid administration

When to consider IV antimicrobials?

• when patient is unable to take PO or unable to absorb the medication
• when you want immediate effect of the medication
• when the spectrum of activity desired is only available with IV antibiotics
• when no PO option is available to treat the pathogen
• when PO medications will not achieve high enough concentrations or penetrations to the location of the infection
  • Critically ill patients; sepsis/bacteremia
  • Endocarditis
  • CNS/ocular infection
  • Osteomyelitis/Septic arthritis (*a study is currently under way, looking at whether certain oral antibiotics are non-inferior to IV antibiotics in
    bone infections⁷)
    *You may occasionally see these syndromes treated with oral antibiotics, because each case is different. But in general, consider these syndromes as ones where IV antibiotics are preferred, especially as initial therapy.

TAKE HOME POINTS:

• IV antimicrobials are NOT “stronger” or “better” than oral antimicrobials
  – it depends on each individual medication
• PO antibiotics should be used unless there is a reason to use IV antibiotics (and not the other way around)
• When PO and IV versions of an antimicrobial are similar, make every concerted effort to make sure your patients are not on IV medications unnecessarily

Questions? Comments? Suggestions for future posts? Leave a comment below.

References:

1. Kwong, L.H et al. (2015). An unsupported preference for intravenous antibiotics. PLoS medicine, 12(5): e1001825. DOI: 10.1371/journal.pmed.1001825
2. MacGregor, R.R. et al. (1997). Oral administration of antibiotics: a rational alternative to the parenteral route. 24: 457-467. PMID: 9114201
3. Chan, R. et al. (1995). Oral versus intravenous antibiotics for community acquired lower respiratory tract infection in a general hospital: open, randomized, controlled trial. BMJ. 310: 1360-1362. PMID: 7787537
4. Baddour et al. (2016). Infective Endocarditis in Adults: Diagnosis, Antimicrobial Therapy, and Management of Complications. Circulation. 132: 1435-1486.
   DOI: 10.1161/CIR.0000000000000296.
5. Tunkel, A.R. et al. (2004). Practice Guidelines for the Management of Bacterial Meningitis. CID. 39: 1267-1284. DOI: 10.1086/425368
6. World Health Organization, Occupational Health. (date published unknown). Comparison of pharmacokinetics and efficacy of oral and injectable medicine [Powerpoint slides]. Retrieved from http://www.who.int/occupational_health/activities/5injvsora.pdf
7. Li, H.K et al. (2015). Oral versus intravenous antibiotic treatment for bone and joint infections (OVIVA): study protocol for a randomized controlled trial. BMC Trials. 16:583. DOI: https://doi.org/10.1186/s13063-015-1098-y
8. Wisplinghoff, H. et al. (2004). Nosocomial bloodstream infections in US hospitals: analysis of 24,179 cases from a prospective nationwide surveillance study. Clin Infect Dis. 39(3): 309-317. DOI: 0.1086/421946

Peer-reviewed by Jeff Pearson, 2nd year PharmD resident