When I was an FNP student, antibiotics felt impossible. There were too many. Each one had different coverage, different dosing, different side effects, different contraindications. And the boards expected me to pick the right one without hesitation.
I had to find a system. Here's the one I built — the one I still use today, and the one I teach.
Antibiotic selection isn't memorizing 50 drugs. It's matching the bug to the drug, then narrowing by patient.
Step 1: Identify the Most Likely Bug
Every infection has a typical pathogen profile. Memorize these:
- Uncomplicated UTI: E. coli (most common), Klebsiella, Proteus, Staph saprophyticus
- Strep throat: Group A Strep (Streptococcus pyogenes)
- Otitis media: Strep pneumoniae, H. influenzae, Moraxella catarrhalis
- Community-acquired pneumonia: Strep pneumoniae, atypicals (Mycoplasma, Chlamydia, Legionella)
- Skin/soft tissue: Staph aureus (consider MRSA), Group A Strep
- Sinusitis (bacterial): Strep pneumoniae, H. influenzae, Moraxella
- Bacterial vaginosis: Gardnerella
- Chlamydia: Chlamydia trachomatis (treat empirically with gonorrhea coverage)
Step 2: Pick the First-Line Drug
Uncomplicated UTI in non-pregnant adult
Nitrofurantoin 100 mg BID x 5 days, OR TMP-SMX DS BID x 3 days (if local resistance <20%), OR fosfomycin 3g single dose. Fluoroquinolones are NOT first-line for uncomplicated UTI.
Strep throat
Penicillin V or amoxicillin x 10 days. If penicillin allergic: cephalexin (low cross-reactivity) or azithromycin/clindamycin.
Otitis media (treat if <2 years, bilateral, severe, or persistent)
Amoxicillin 80–90 mg/kg/day x 10 days. If recent antibiotics or failure: amoxicillin-clavulanate.
Community-acquired pneumonia (outpatient, healthy)
Amoxicillin 1g TID OR doxycycline OR a macrolide (azithromycin) if local resistance <25%. If comorbidities: respiratory fluoroquinolone (levofloxacin) OR combination beta-lactam + macrolide.
Skin/soft tissue (non-purulent)
Cephalexin or dicloxacillin. If suspect MRSA (purulent, recent abscess, MRSA history): TMP-SMX, doxycycline, or clindamycin.
Bacterial sinusitis (only if >10 days symptoms or worsening)
Amoxicillin-clavulanate 875/125 mg BID x 5–7 days.
Chlamydia
Doxycycline 100 mg BID x 7 days, OR azithromycin 1g single dose. Treat partners. Consider co-treatment for gonorrhea.
Gonorrhea
Ceftriaxone 500 mg IM single dose. Co-treat chlamydia.
Test Pearl: If a question asks you to "treat empirically," that means before culture results. Pick the drug that covers the most likely organism without overusing broad-spectrum agents.
Step 3: Adjust for the Patient
- Pregnant: Avoid tetracyclines, fluoroquinolones, sulfonamides in third trimester, nitrofurantoin near term. Penicillins and cephalosporins are usually safe.
- Renal impairment: Dose-adjust fluoroquinolones, aminoglycosides, vancomycin. Avoid nitrofurantoin if CrCl < 30.
- Penicillin allergy: Type and severity matter. True anaphylaxis = avoid penicillins and cephalosporins. Mild rash = cephalosporins usually fine.
- Children < 8: Avoid tetracyclines (teeth staining), fluoroquinolones (cartilage concerns).
The Wrong Answers
The FNP boards loves these distractors:
- Treating viral URIs or viral pharyngitis with antibiotics — wrong.
- Treating asymptomatic bacteriuria in non-pregnant adults — usually wrong.
- Picking a fluoroquinolone for an uncomplicated UTI — wrong.
- Treating before culture in cases where culture is mandatory (suspected MRSA, recurrent UTI in a man) — wrong.
The System
For every infection question on the exam, do this in your head:
- Name the bug.
- Name the first-line drug.
- Check for allergies, pregnancy, age, kidney function.
- Pick.
Once this is automatic, antibiotic questions become some of your fastest points.