Ampicillin-Sulbactam (Unasyn)
ampicillin sodium / sulbactam sodium
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Skin and skin structure infections (cellulitis, cutaneous abscesses, wound infections) | Adults and pediatric ≥1 year | Monotherapy | FDA Approved |
| Intra-abdominal infections (peritonitis, intra-abdominal abscess) | Adults | Monotherapy | FDA Approved |
| Gynecological infections (endometritis, PID) | Adults | Monotherapy | FDA Approved |
Ampicillin-sulbactam combines an aminopenicillin with a beta-lactamase inhibitor, restoring activity against many beta-lactamase-producing organisms including S. aureus (MSSA), E. coli, Klebsiella spp., H. influenzae, Bacteroides fragilis, and Acinetobacter species. It retains ampicillin’s inherent activity against enterococci, streptococci, and Listeria. It does not cover MRSA, Pseudomonas aeruginosa, or ESBL-producing Enterobacterales.
Community-acquired aspiration pneumonia: Frequently used for polymicrobial aspiration pneumonia due to excellent anaerobic coverage. Evidence quality: Moderate.
Human and animal bite wounds: Recommended by IDSA guidelines for bite-wound infections covering oral flora including Pasteurella, Eikenella, and anaerobes. Evidence quality: High.
Deep neck space infections (Ludwig’s angina, peritonsillar abscess): Commonly used as empiric therapy for polymicrobial head and neck infections. Evidence quality: Moderate.
Carbapenem-resistant Acinetobacter baumannii (CRAB): IDSA 2022 guidance recommends high-dose ampicillin-sulbactam (9 g IV q8h over 4-hour infusion) as sulbactam has intrinsic activity against Acinetobacter via PBP binding. Evidence quality: Moderate.
Diabetic foot infections (mild-moderate): Covers polymicrobial flora including MSSA, streptococci, and anaerobes. Evidence quality: Moderate.
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Skin / soft tissue infection (mild–moderate) | 1.5–3 g IV/IM q6h | 1.5–3 g IV/IM q6h | 12 g/day (sulbactam max 4 g/day) | Duration 7–14 days IV over 10–15 min (bolus) or 15–30 min (infusion); IM via deep injection |
| Intra-abdominal infection (complicated appendicitis, peritonitis) | 3 g IV q6h | 3 g IV q6h | 12 g/day | Duration 5–14 days depending on source control Use higher dose (3 g) for serious infections |
| Gynecological infection (endometritis, PID) | 3 g IV q6h | 3 g IV q6h | 12 g/day | Duration guided by clinical response Consider adding doxycycline for PID per CDC guidelines |
| Bite wound infection (human/animal) — off-label | 1.5–3 g IV q6h | 1.5–3 g IV q6h | 12 g/day | Step down to oral amoxicillin-clavulanate when clinically appropriate Covers Pasteurella, Eikenella, oral anaerobes |
| Aspiration pneumonia — off-label | 3 g IV q6h | 3 g IV q6h | 12 g/day | Duration 5–7 days typically Excellent anaerobic coverage for aspiration |
| CRAB infection — high-dose sulbactam (off-label) | 9 g IV q8h | 9 g IV q8h | 27 g/day | Infuse over 4 hours; exceeds standard sulbactam cap Per IDSA 2022 CRAB guidance; sulbactam provides PBP-mediated activity against A. baumannii |
Renal Impairment Dosing — Adults (FDA PI)
| CrCl (mL/min) | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| ≥30 | 1.5–3 g q6–8h | 1.5–3 g q6–8h | 12 g/day | No dose adjustment required |
| 15–29 | 1.5–3 g q12h | 1.5–3 g q12h | 6 g/day | Extend interval |
| 5–14 | 1.5–3 g q24h | 1.5–3 g q24h | 3 g/day | Extend interval further |
| Hemodialysis | 1.5–3 g q24h | 1.5–3 g q24h | 3 g/day | Administer after dialysis on dialysis days Both agents removed by hemodialysis |
Pediatric Dosing (≥1 year, <40 kg)
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Skin and skin structure infections (FDA-approved) | 300 mg/kg/day IV divided q6h | 300 mg/kg/day IV divided q6h | Adult dose if ≥40 kg | = 200 mg/kg ampicillin + 100 mg/kg sulbactam per day IV only for pediatrics; max treatment 14 days IV, then switch to oral |
The FDA-approved maximum sulbactam dose is 4 g/day (corresponding to 12 g/day total Unasyn). However, for CRAB infections, the IDSA 2022 guidance recommends doses far exceeding this limit — 9 g total (6 g ampicillin + 3 g sulbactam) every 8 hours, infused over 4 hours. This high-dose strategy leverages sulbactam’s intrinsic bactericidal activity against A. baumannii via binding to PBP1 and PBP3, independent of beta-lactamase inhibition. This is a fundamentally different pharmacological rationale from standard use.
Pharmacology
Mechanism of Action
Ampicillin is an aminopenicillin that exerts bactericidal activity by binding to penicillin-binding proteins (PBPs) in the bacterial cell wall, inhibiting the final transpeptidation step of peptidoglycan synthesis. This leads to cell wall weakening, osmotic instability, and bacterial lysis during active growth. Ampicillin has broad activity against gram-positive organisms (streptococci, enterococci, Listeria), many gram-negative aerobes (E. coli, Proteus mirabilis, H. influenzae), and anaerobes. However, ampicillin is readily hydrolysed by beta-lactamases, limiting its utility against resistant organisms. Sulbactam is a penicillanic acid sulfone that irreversibly inhibits many plasmid-mediated and chromosomal class A beta-lactamases, preventing them from inactivating ampicillin. While sulbactam has minimal intrinsic antibacterial activity against most organisms, it does possess clinically meaningful direct activity against Neisseria species and, importantly, against Acinetobacter baumannii through direct PBP binding — a property exploited in high-dose CRAB protocols.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | IV: peak levels immediately post-infusion (amp 109–150 mcg/mL at 2 g dose); IM: peak amp 8–37 mcg/mL | IM route available for outpatient settings; IV preferred for serious infections |
| Distribution | Amp 28% protein bound; sulb 38%; good penetration to peritoneal fluid, intestinal mucosa, appendix, blister fluid; CSF penetration with inflamed meninges | Tissue levels generally adequate for intra-abdominal and soft tissue infections; can reach CSF in meningitis |
| Metabolism | Minimal; both agents undergo negligible hepatic metabolism | No hepatic dose adjustment; however, monitor LFTs given hepatotoxicity risk |
| Elimination | t½ ~1 h (both agents); 75–85% excreted unchanged in urine within 8 h; both removable by hemodialysis | Renal dose adjustment required when CrCl <30 mL/min; dose after dialysis |
Side Effects
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Pain at IM injection site | 16% | Expected with IM administration; can be reduced by using lidocaine as diluent where permitted |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Diarrhea | 3% | Most common systemic adverse reaction; usually mild; evaluate for C. difficile if severe or bloody |
| Pain at IV injection site | 3% | Can be minimized by slower infusion and adequate dilution |
| Thrombophlebitis | 3% | Rotate IV sites; consider midline or PICC for prolonged courses |
| Rash | <2% | Distinguish allergic rash from non-allergic ampicillin rash (5–10% in children, especially with mononucleosis); non-allergic rash is dull red, maculopapular, appears day 3–14 |
| Phlebitis | 1.2% | Related to infusion; use adequate dilution volume |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Anaphylaxis | Rare | Minutes to first hour | Stop immediately; epinephrine; airway management; permanent discontinuation |
| Stevens-Johnson syndrome / TEN | Very rare | 1–4 weeks | Discontinue immediately; dermatology consultation; supportive care |
| DRESS | Very rare | 2–8 weeks | Discontinue; systemic corticosteroids; monitor end-organ function |
| Hepatotoxicity (cholestatic jaundice, hepatitis) | Uncommon; deaths reported | Days to weeks | Monitor LFTs; discontinue if jaundice or significant transaminase elevation; contraindicated on re-exposure |
| Clostridioides difficile–associated diarrhea | Uncommon | During therapy to ≥2 months after | Test for toxin; discontinue ampicillin-sulbactam if possible; treat with vancomycin or fidaxomicin per guidelines |
| Hemolytic anemia | Very rare (postmarketing) | Variable | Discontinue; supportive care; usually reversible |
| Agranulocytosis / thrombocytopenic purpura | Very rare (postmarketing) | Typically with prolonged therapy | Discontinue; monitor CBC; usually reversible |
| Seizures | Very rare; higher risk with renal impairment | Variable; dose-related | Ensure appropriate renal dosing; benzodiazepines for acute seizure |
| Acute generalized exanthematous pustulosis (AGEP) | Very rare (postmarketing) | Days to weeks | Discontinue; dermatology consultation; usually self-resolving after cessation |
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| Rash / dermatologic reactions | <2% | Evaluate for allergic vs non-allergic ampicillin rash |
| GI intolerance (diarrhea, nausea) | ~3% affected; rarely requires cessation | Usually mild and self-limiting |
| Hepatotoxicity | Rare | Warrants permanent discontinuation; re-exposure is contraindicated |
A dull, red maculopapular rash occurs in 5–10% of children receiving ampicillin-class drugs, typically appearing on day 3–14 of therapy. This non-allergic rash is particularly common in patients with infectious mononucleosis (up to 70–100% incidence), in whom ampicillin-class agents should be avoided. The rash is not a true allergy and does not preclude future penicillin use, but careful clinical evaluation is needed to distinguish it from urticarial or anaphylactic hypersensitivity.
Drug Interactions
Ampicillin-sulbactam undergoes minimal hepatic metabolism and is not a significant CYP450 inducer or inhibitor. Its key interactions relate to renal excretion competition, physical incompatibility with aminoglycosides, and pharmacodynamic effects on coagulation and gut flora.
Monitoring
- Hepatic FunctionBaseline; periodically during therapy
RoutineAST, ALT, alkaline phosphatase, bilirubin. Hepatotoxicity including cholestatic jaundice has been reported with deaths; monitor at regular intervals, especially in patients with pre-existing hepatic impairment - Renal FunctionBaseline; then as indicated
RoutineBUN, creatinine; adjust dosing interval when CrCl <30 mL/min; increases in BUN and creatinine reported in clinical trials - CBCBaseline; weekly if >7 days
RoutineMonitor for leukopenia, neutropenia, thrombocytopenia, eosinophilia; agranulocytosis and hemolytic anemia reported in postmarketing - Signs of Allergy / SkinEach dose; ongoing
RoutineFirst dose: monitor for anaphylaxis. Ongoing: assess for progressive rash, mucosal involvement (SJS/TEN), or systemic symptoms (DRESS). Discontinue for progressive lesions - GI SymptomsOngoing; test if severe diarrhea
Trigger-basedTest for C. difficile toxin if diarrhea is watery, bloody, or persistent; can occur up to 2+ months post-treatment - SuperinfectionOngoing
Trigger-basedMonitor for oral or vaginal candidiasis and resistant bacterial superinfection during prolonged courses
Contraindications & Cautions
Absolute Contraindications
- History of serious hypersensitivity (anaphylaxis, SJS) to ampicillin, sulbactam, any penicillin, or other beta-lactam antibacterials (including cephalosporins)
- Previous cholestatic jaundice or hepatic dysfunction associated with ampicillin-sulbactam use — re-exposure is specifically contraindicated per FDA PI
Relative Contraindications (Specialist Input Recommended)
- Infectious mononucleosis: A high percentage of patients with mononucleosis develop a generalized maculopapular rash when given ampicillin-class agents; avoid use in known or suspected mono
- Known cephalosporin allergy (severe): Cross-reactivity estimated at 1–2%; specialist assessment recommended for severe previous cephalosporin reactions
Use with Caution
- Renal impairment (CrCl <30 mL/min): Dose interval extension required; both agents accumulate; increased risk of neurotoxicity (seizures) at elevated drug concentrations
- Pre-existing hepatic disease: Monitor hepatic function at regular intervals; hepatotoxicity (cholestatic jaundice) has resulted in fatal outcomes
- Concurrent allopurinol therapy: Substantially increased rash incidence
- Patients on oral anticoagulants: Monitor coagulation more frequently
- Sodium-restricted diets: Significant sodium load (5 mEq per 1.5 g; up to 40 mEq/day at full dose of 12 g/day)
Hepatic dysfunction, including hepatitis and cholestatic jaundice, has been associated with ampicillin-sulbactam use. While usually reversible, fatal cases have been reported. Hepatic function should be monitored at regular intervals in patients with hepatic impairment. A previous episode of cholestatic jaundice or hepatic dysfunction attributable to ampicillin-sulbactam is a contraindication to re-exposure.
Ampicillin-sulbactam has been associated with severe skin reactions including toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome (SJS), drug reaction with eosinophilia and systemic symptoms (DRESS), dermatitis exfoliative, erythema multiforme, and acute generalized exanthematous pustulosis (AGEP). Any progressive skin rash should prompt immediate discontinuation and evaluation.
Patient Counselling
Purpose of Therapy
Ampicillin-sulbactam is an antibiotic given through a vein or by injection to treat serious bacterial infections. It works by killing bacteria and preventing them from resisting treatment. It is not effective against viral infections such as the common cold or influenza.
How to Take
This medication is administered by healthcare professionals, usually every 6 hours by IV infusion (over 15–30 minutes) or by deep injection into a muscle. Each scheduled dose is important — completing the full course is essential even if symptoms improve early, as stopping prematurely increases the risk of resistant infections.
Sources
- Unasyn (ampicillin sodium and sulbactam sodium) for Injection. Full Prescribing Information. Pfizer Inc. Revised May 2024. FDA Label (PDF) Primary source for all approved indications, dosing, renal adjustment, adverse reactions, contraindications, and drug interactions.
- Unasyn (ampicillin sodium and sulbactam sodium). DailyMed label. National Library of Medicine. DailyMed Current NLM-hosted label with full prescribing information and clinical pharmacology data.
- Unasyn (ampicillin sodium and sulbactam sodium) for Injection. Updated Prescribing Information January 2025. FDA Label 2025 (PDF) Most recent FDA label update with current warnings including severe cutaneous adverse reactions.
- Yellin AE, Heseltine PN, Berne TV, et al. The role of sulbactam/ampicillin in the treatment of serious surgical infections. Surg Gynecol Obstet. 1993;177(Suppl):30-40. Pivotal trial supporting efficacy of ampicillin-sulbactam in intra-abdominal and surgical infections.
- Betrosian AP, Frantzeskaki F, Xanthaki A, Georgiadis G. High-dose ampicillin-sulbactam as an alternative treatment of late-onset VAP from multidrug-resistant Acinetobacter baumannii. Scand J Infect Dis. 2007;39(1):38-43. doi:10.1080/00365540600951184 Evidence supporting high-dose sulbactam strategy for MDR Acinetobacter infections.
- Tamma PD, Aitken SL, Bonomo RA, Mathers AJ, van Duin D, Clancy CJ. IDSA 2023 guidance on the treatment of antimicrobial-resistant gram-negative infections. Clin Infect Dis. 2023. doi:10.1093/cid/ciad428 IDSA guidance recommending high-dose ampicillin-sulbactam (9 g q8h over 4 h) for CRAB infections.
- Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. Clin Infect Dis. 2014;59(2):e10-52. doi:10.1093/cid/ciu296 IDSA guidelines listing ampicillin-sulbactam as an option for bite wounds and polymicrobial soft tissue infections.
- Solomkin JS, Mazuski JE, Bradley JS, et al. Diagnosis and management of complicated intra-abdominal infection in adults and children: guidelines by SIS and IDSA. Clin Infect Dis. 2010;50(2):133-164. doi:10.1086/649554 Recommends ampicillin-sulbactam as one option for community-acquired intra-abdominal infections of mild-moderate severity.
- Bush K, Jacoby GA. Updated functional classification of beta-lactamases. Antimicrob Agents Chemother. 2010;54(3):969-976. doi:10.1128/AAC.01009-09 Comprehensive beta-lactamase classification relevant to sulbactam’s spectrum of inhibition.
- Penwell WF, Shapiro AB, Giacobbe RA, et al. Molecular mechanisms of sulbactam antibacterial activity and resistance determinants in Acinetobacter baumannii. Antimicrob Agents Chemother. 2015;59(3):1680-1689. doi:10.1128/AAC.04808-14 Elucidates sulbactam’s direct PBP-mediated bactericidal activity against A. baumannii, the basis for high-dose CRAB protocols.
- Peechakara BV, Gupta M. Ampicillin/Sulbactam. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan. Updated January 11, 2024. NCBI Bookshelf Comprehensive review covering pharmacology, indications, adverse effects, and drug interactions with current references.
- Ito A, Kohno S, Nishizawa K, et al. Population pharmacokinetics of ampicillin and sulbactam in patients with community-acquired pneumonia: evaluation of the impact of renal impairment. Br J Clin Pharmacol. 2015;79(4):681-691. doi:10.1111/bcp.12533 Population PK model supporting renal dose adjustments and time-above-MIC analysis for various degrees of renal impairment.