October 22nd, 2010

How to Figure Out the Length of Antibiotic Therapy

keys to the kingdomOne thing we ID doctors know — that other clinicians simply don’t — is how long to treat a patient with antibiotics.

I was reminded of this special power by these recent events:

  • An excellent fellow from the hospital’s Critical Care program rotated through our division recently.  When asked about what she wanted learn from the elective, the first thing she said was figuring out — finally — how long an antibiotic course should be.
  • We received a consultation on an unfortunate case that had been hospitalized since approximately the time of the Gerald Ford administration.  You know the type — multiple surgeries gone bad, several GI anastomotic leaks, innumerable tubes and drains, abundant highly-resistant bacteria and yeast cultured from each drain.  The question:  “How long should we treat the acinetobacter?”
  • This great article in the New York Times by Harvard “Happiness” Professor Daniel Gilbert, who relays being given a 7-day course of antibiotics by a doctor.  “I understood why I needed to complete the full course, of course. What I didn’t understand was why a full course took precisely seven days. Why not six, eight or nine and a half? Did the number seven correspond to some biological fact about the human digestive tract or the life cycle of bacteria?”

The answer, Dr. Gilbert, is that this is highly-specialized knowledge, rarefied information that only 100% Board-Certified, USDA-inspected Infectious Diseases Doctors know.  And since I’m concerned that your article might give readers the wrong impression about our scientific credibility, I’ll now divulge what we’ve learned, and how to apply it.

To figure out how long antibiotics need to be given, use the following rules:

  1. Choose a multiple of 5 (fingers of the hand) or 7 (days of the week).
  2. Is it an outpatient problem that is relatively mild?  If so, choose something less than 10 days.  After application of our multiples rule, this should be 5 or 7 days.
  3. Is it really mild, so much so that antibiotics probably aren’t needed at all but clinician or patient are insistent?  Break the 5/7 rule and go with 3 days.  Ditto uncomplicated cystitis in young women.
  4. Is it a serious problem that occurs in the hospital or could end up leading to hospitalization?  With the exception of community-acquired pneumonia (5 or 7 days), 10 days is the minimum.
  5. Patient not doing better at the end of some course of therapy?  Extend treatment, again using a multiple of 5 or 7 days.
  6. Does the infection involve a bone or a heart valve?  Four weeks (28 days) at least, often 6 weeks (42 days).  Note that 5 weeks (35 days) is not an option — here the 5’s and 7’s cancel each other out, and chaos ensues.
  7. The following lengths of therapy are inherently weird, and should generally be avoided:  2, 4, 6, 8, 9, 11, 12, 13 days.  Also, 3.14159265 days.

In this highly data-driven exercise, it is imporant also to note the number of rules — seven, as in days of the week.

That did not occur by chance.

7 Responses to “How to Figure Out the Length of Antibiotic Therapy”

  1. Mala Gupta says:


  2. anonydoc says:

    1. great article.
    2. interestingly, whatever short-time treatments i recommend my patients, not matter wheter it’s pharmcotherapy or something else, i usually recommed it for 3-5, 7, 10 or 14 days, or rarely 21 or 4-6(-8) weeks…
    3. “How long should we treat the acinetobacter?” — please give us the answer!


    • Paul Sax says:

      Dear Anonydoc,
      1. Thanks!
      2. Remember, never use 5 weeks!
      3. “Long enough” — so long as it follows the rules.

  3. Richard Teplick says:

    The most important rule is to use only prime numbers so the length of cannot be divided by any number except itself

  4. Rodolfo Ochoa says:

    Excellent, Dr. Sax!!!

  5. O. RN. says:

    I haven’t read the original article referenced above, but I certainly hope this is a tongue in cheek response, and not a Doc voicing the all too common, “It is so because I say so, and evidence based practice be damned” answer. Forgive me, but I hear that reply every day, not just to patients, but to other Doctors and Nurses as well. Maybe I still don’t get “Professional” humor.

  6. […] Paul E. Sax, NEJM Journal Watch HIV/AIDS Clinical Care, October 22nd 2010 […]

HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

Contributing Editor

NEJM Journal Watch
Infectious Diseases

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