Advanced Pharmacology · Topic 13 of 13

Antibiotic Pharmacology

Pharmacology · antibacterial drugs

Antibiotic Pharmacology

Dose by how each class kills: vancomycin to an AUC of 400–600, aminoglycosides once daily, beta-lactams by time above MIC — and know the toxicities and true allergy risk.

Key numbers

Vancomycin: target AUC 400–600AUC 400–600 mg·h/L

Once-daily gentamicin 5–7 mg/kgtrough < 1 mcg/mL

Load vancomycin 20–35 mg/kg (max 3 g)20–35 mg/kg

Vancomycin flushing: slow the infusion≤ 10 mg/min

01 · WatchVancomycin: target AUC 400–600

For serious MRSA infection, dose vancomycin to an AUC of 400–600 mg·h/L (MIC 1), using Bayesian software with 1–2 levels or a peak plus trough; 15–20 mg/L trough targets are abandoned — they caused more nephrotoxicity.

02 · AvoidFluoroquinolone boxed warning: tendon, nerve, CNS

Fluoroquinolones carry a boxed warning for disabling, potentially irreversible tendinitis and tendon rupture, peripheral neuropathy and CNS effects, and they can worsen myasthenia gravis — avoid them in myasthenia.

03 · The rulePenicillin allergy rarely rules out cephalosporins

With an unconfirmed, nonanaphylactic penicillin allergy, any cephalosporin can be given without testing; after penicillin anaphylaxis, give a cephalosporin with a dissimilar side chain — cefazolin's side chains are unique, so cross-reactivity is very low.

What it is

Core idea

Beta-lactams kill by time above MICBeta-lactam killing depends on time above the MIC, not peak height; in sepsis or septic shock give maintenance doses as prolonged infusions after a loading dose (strong recommendation, SSC 2026).

Causes & risk factors

Why it happens

Tendon rupture: age > 60, steroids, transplantFluoroquinolone tendon rupture risk is highest over age 60, with corticosteroids, and after kidney, heart or lung transplant; it can be bilateral and occur up to months after therapy.

Management

Orders

Once-daily gentamicin 5–7 mg/kgAminoglycosides kill by peak concentration while toxicity tracks exposure time, so give them once daily: gentamicin or tobramycin 5 mg/kg (7 mg/kg if critically ill), amikacin 15 mg/kg; once-daily trough should be < 1 mcg/mL.

Load vancomycin 20–35 mg/kg (max 3 g)Give a vancomycin loading dose by actual body weight to critically ill patients and those on renal replacement or continuous infusion: 20–35 mg/kg (maximum 3,000 mg), or 20–25 mg/kg in obesity.

Red flags & don'ts

Avoid harm

Aminoglycosides: kidney and inner-ear toxicity complicationAminoglycosides cause nephrotoxicity (often reversible), vestibular and cochlear ototoxicity (often irreversible) and prolonged neuromuscular blockade; risk rises with therapy > 3 days, high levels, older age, kidney disease and other nephrotoxins.

Vancomycin flushing: slow the infusion complicationVancomycin infusion reaction — pruritus and flushing of the face, neck and shoulders — is histamine release, not allergy: infuse over at least 60 minutes, no faster than 10 mg/min.

Vancomycin nephrotoxicity rises with co-nephrotoxins interactionVancomycin nephrotoxicity is uncommon unless exposure is high or other nephrotoxins are given — aminoglycosides, and possibly piperacillin-tazobactam; reduce doses in renal insufficiency.

Fluoroquinolones: QT, glucose, aorta, C. difficile complicationFluoroquinolones also prolong the QT interval, cause hypo- or hyperglycemia (especially with diabetes drugs), raise aortic aneurysm and dissection risk and strongly predispose to C. difficile; separate oral doses from calcium, magnesium, aluminum, zinc and iron.

Cefepime neurotoxicity in renal impairment complicationCefepime can cause encephalopathy, myoclonus, seizures and nonconvulsive status epilepticus, mostly in renal impairment without dose adjustment; high-dose penicillins and imipenem can also cause seizures in renal insufficiency.

The rules

Know these

Reserve fluoroquinolones for no-alternative casesBecause of serious adverse reactions, reserve fluoroquinolones for uncomplicated UTI, acute bacterial sinusitis and acute exacerbation of chronic bronchitis only when no alternative exists (FDA labeling).

Carbapenems and aztreonam in beta-lactam allergyWith penicillin or cephalosporin allergy, a carbapenem may be given without testing or extra precautions, and aztreonam too — unless the allergy is to ceftazidime, which shares its side chain (2022 drug allergy parameter).

Delabel with a direct amoxicillin challengeAbout 10% of people report penicillin allergy but most tolerate re-exposure; adults with a distant (> 5 years) benign cutaneous reaction can have a direct oral amoxicillin challenge without skin testing.

Beta-lactam or aminoglycoside dosing?

Commonly confused

Killing pattern	Time-dependent: time above MIC
Dosing strategy	Frequent doses or prolonged infusions (sepsis)
Monitoring	Clinical response; dose for renal function
Main toxicity	Hypersensitivity; neurotoxicity at high doses in renal failure

More detail

Reference

Show 6 reference facts

About 2% of penicillin-allergic patients cross-react with cephalosporins; risk depends on shared R1 side chains, and third- or fourth-generation cephalosporins with dissimilar side chains carry negligible risk.

Aminoglycoside targets: gentamicin or tobramycin peak 15–20 mcg/mL (16–24 if critically ill), at least 10 × MIC; with traditional dosing, trough 1–2 mcg/mL. Check creatinine every 2–3 days and audiography if therapy exceeds 2 weeks.

Dose aminoglycosides on ideal body weight when actual weight is up to 120% of IBW, adjusted body weight [IBW + 0.4 × (actual − IBW)] above that, and actual weight if below IBW.

A continuous vancomycin infusion titrated to a steady-state level of 17–25 mcg/mL is an alternative to intermittent AUC dosing; AUC-based recommendations apply to MRSA, not other gram-positive infections.

After penicillin anaphylaxis, avoid penicillins and beta-lactams with similar side chains unless an allergist clears them (penicillin skin testing is suggested); when no substitute exists, give the drug by desensitization (induction of tolerance).

Aminoglycosides, fluoroquinolones and daptomycin kill in a concentration-dependent way; aminoglycosides and fluoroquinolones work best as intermittent boluses reaching peaks ≥ 10 × MIC, while vancomycin activity tracks AUC/MIC.

Pearl

A penicillin allergy label rarely means no cephalosporin — and cefazolin's unique side chain makes it the safest bet.

Sources · IDSA — Therapeutic Monitoring of Vancomycin for Serious MRSA Infections: Revised Consensus Guideline (ASHP/IDSA/PIDS/SIDP, Rybak MJ et al., Am J Health-Syst Pharm 2020) · Merck Manual Professional — Vancomycin · DoseMe — Vancomycin dosing guidelines essentials (2020 consensus: loading 20–35 mg/kg, max 3000 mg; obesity 20–25 mg/kg) · Merck Manual Professional — Aminoglycosides · Merck Manual Professional — Fluoroquinolones · Levofloxacin tablets prescribing information (boxed warning, tendon rupture risk) — DailyMed · Brameli A, Stone CA, Phillips EJ. Evaluation of cephalosporin allergy: survey of drug allergy experts (summarizes the 2022 AAAAI/ACAAI drug allergy practice parameter). J Allergy Clin Immunol Glob 2025 · Merck Manual Professional — Cephalosporins (cross-reactivity, cefazolin side chain) · Merck Manual Professional — Overview of Antibacterial Medications (pharmacodynamics) · SCCM — Surviving Sepsis Campaign International Guidelines for Management of Sepsis and Septic Shock 2026 (recommendation list) · University of Iowa Internal Medicine — Updates on Antibiotic Drug Allergy (2023; quotes Khan DA et al., Drug allergy: a 2022 practice parameter update, J Allergy Clin Immunol 2022;150:1333–93) · Merck Manual Professional — Penicillins (hypersensitivity, CNS toxicity) · Cefepime for injection prescribing information (5.2 Neurotoxicity) — DailyMed · Merck Manual Professional — Carbapenems (imipenem seizures) · UNMC Infectious Diseases blog — Addressing reported penicillin allergy: a 2022 practice parameter update (penicillin allergy reported by up to 10% of the population, 16% of inpatients) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).