MD Notes

creation date: 2026-09-03 14:32
tags: Interventions

Prescribing Antibiotics

The approach to antibiotics use is described here. For antibiotic classes, see here.

Necessity of Antibiotics

The patient should have an infection that require antibiotic use. Confirm a infectious diagnosis, especially in cases refractory to treatment.

When counselling patients, the following points may aid in convincing them antibiotics are unnecessary. Inappropriate antibiotics use can lead to:

  • Adverse reactions and side effects
  • Secondary infections (incl. C. difficile)
  • Promotion of drug-resistant organisms

Microbes / Organisms

Determination of organisms can be made via culture or treatment can be empiric.

Cultures are indicated for the following:

  • Sputum: severe CAP, previous MRSA/Pseudomonas, recent abx (past 90 days)
  • Urine: pyelonephritis, recurrent cystitis, pregnancy, complicated urologic cases, pre-urologic procedure with bleeding
    • Never send if asymptomatic (no irritative symptoms)
    • For foley catheters, look for purulent discharge around catheter
  • Blood: suspected bacteremia (high risk infections such as meningitis, epidural abscesses, nontraumatic septic arthritis; instrumentation and prosthetics )
    • Note risk of contamination (3-5%) leading to unnecessary treatment
  • Skin swab: purulent skin infections, concern for MRSA

By Gram Stain

Gram Positive Cocci

Gram Positive Bacilli

Gram Negative Bacilli

Non-Gram Staining Bacteria

By Infection Site

Antibiotic Options

Skin and Soft Tissue Infection

Mild-moderate, purulent or non-purulent can be treated outpatient:

  • Cephalexin or cefadroxil (same coverage except cephalexin QID vs cefadroxil BID (preferred))
  • Cefuroxime if penicillin allergy
  • TMP-SMX if MRSA risk factors

Cystitis (Uncomplicated)

Treated outpatient:

  • Refer to previous cultures for resistance patterns
  • First line:
    • Nitrofurantoin x 5 days
    • TMP-SMX x 3 days
    • Fosfomycin x 1-2 doses (less effective)
  • Second line:
    • Ciprofloxacin
    • Beta-lactams

Community Acquired Pneumonia

For a healthy patient, outpatient treatment is:

  • Amoxicillin
  • Doxycycline (note: teratogen)
  • Azithromycin (in some communities, depending on biogram)

For patient with comorbidities, outpatient treatment is:

  • Amoxicillin with clauvlanic acid
  • Cefuroxime PLUS doxycycline or azithromycin (for legionella/atypical)
  • Levofloxacin/moxifloxacin (one-drug option, more side effects)

Sepsis

Options vary based on most likely source of infection.
CAP

  • Ceftriaxone 2g IV + azithromycin 500 mg PO/IV
  • Levofloxacin 750mg PO/IV or moxifloxacin 400mg PO/IV

UTI

  • Ceftriaxone 2g IV
  • Ertapenem 1g IV (if resistance suspected)

Intraabdominal Infection

  • Ceftriaxone 2g IV + metronidazole 500mg PO/IV
  • Amoxicillin-clavulanate 2000mg/200mg IV
  • Piperacillin-tazobactam 3.375g IV
  • Ciprofloxacin 400mg PO/IV + metronidazole 500mg PO/IV

Acute soft-tissue/bone/joint

  • Cefazolin 2g IV

Infective endocarditis

  • Vancomycin 20 mg/kg IV + ceftriaxone 2g IV

Central venous catheter

  • Vancomycin 20 mg/kg IV + ceftazidime 2g IV + remove line

Necrotizing fasciitis (type 1 - polymicrobial)

  • Piperacillin-tazobactam 4.5 g IV
  • Clindamycin 900 mg IV

Necrotizing fasciitis (type 2 - monomicrobial)

  • Penicillin G 4M IU IV + clindamycin 900mg IV

Diabetic foot infection (localized, no high-risk factors)

  • Cefazolin 2g IV

Febrile neutropenia

  • Piperacillin-tazobactam 4.5 g IV

Meningitis

  • FIRST: dexamethasone 10 mg
  • THEN ceftriaxone 2g IV or vancomycin 20 mg/kg IV
  • ± ampicillin 2g IV

Antibiotic Route / Duration / De-escalation

Reassess the situation:

  • Review initial diagnosis / clinical status
  • Narrow antimicrobials as appropriate based on culture results

Change IV to PO if:

  • Vital signs normalizes
  • Condition does not need longer IV treatment
  • Able to swallow and absorb medications
  • No resistance needing IV therapy

Many antibiotics have good bioavailability (equivalence PO to IV). Examples:

  • Amoxicillin 250-1000mg q8h
  • Clindamycin 300-600mg q8h
  • Ciprofloxacin 250-750mg q12h
  • Doxycycline 100mg q12h
  • Levofloxacin 500-750mg q24h
  • Moxifloxacin 400mg q24h
  • Linezolid 600mg q12h
  • TMP-SMX 160/800mg q12h
  • Metronidazole 500mg q12h

Antibiotics can often be prescribed with shorter durations with the same effectiveness:

  • CAP: 5 days (3 if symptoms resolved)
  • SSTI: 5 days
  • UTI 3 days (septra/cipro), 5 days other abx

Exceptions to shorter duration are:

  • Otitis media for <2 years old
  • Strep throat with penicillin

References

Tools / Guidelines

Additional Reading