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Carbapenem antibiotic Pregnancy: Available data from a small number of postmarketing cases are not sufficient to identify drug-associated risks for major birth defects, miscarriage, or adverse maternal or fetal outcomes. Developmental toxicity studies in mice, rats, rabbits and monkeys at 0.4 to 2.9 times the recommended human dose showed no drug-induced fetal malformations; embryofetal studies in cynomolgus monkeys at doses similar to the recommended human dose showed an increase in embryonic loss. Lactation: insufficient data on the presence of imipenem/cilastatin in human milk, the effects on the breastfed child, or milk production; the developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for the drug and any potential adverse effects on the breastfed child.

Imipenem with cilastatin

Brand names: Primaxin

Imipenem with cilastatin is a broad-spectrum intravenous carbapenem antibiotic combination used for serious infections, including hospital-acquired and polymicrobial infections caused by susceptible organisms.

Adult dose

Dose: Serious infections caused by susceptible bacteria (lower respiratory tract, urinary tract, intra-abdominal, gynaecologic, bacterial septicaemia, bone and joint, skin and skin structure infections, endocarditis), adults with creatinine clearance 90 mL/min or more: infection due to a susceptible organism - 500 mg every 6 hours OR 1000 mg every 8 hours; infection due to an organism with intermediate susceptibility - 1000 mg every 6 hours. Doses are expressed as the quantity of imipenem; an equivalent amount of cilastatin is also present.
Route: Intravenous infusion only (500 mg over 20 to 30 minutes; 1000 mg over 40 to 60 minutes)
Frequency: Every 6 hours or every 8 hours, depending on regimen and pathogen susceptibility
Max: 4 g/day (label: maximum total daily dosage should not exceed 4 g/day)
Table 1 (adults, creatinine clearance 90 mL/min or more): 'If the infection is suspected or proven to be due to a susceptible bacterial species: 500 mg every 6 hours OR 1000 mg every 8 hours. If the infection is suspected or proven to be due to bacterial species with intermediate susceptibility: 1000 mg every 6 hours.' 'The dosage of Imipenem and Cilastatin for Injection in adult patients should be based on suspected or confirmed pathogen susceptibility.' 'A reduction in dose must be made for patients with creatinine clearance less than 90 mL/min' (see renal adjustment). 'Administer 500 mg by intravenous infusion over 20 to 30 minutes. Administer 1000 mg by intravenous infusion over 40 to 60 minutes. In patients who develop nausea during the infusion, the rate of infusion may be slowed.' The reconstituted suspension must not be administered by direct intravenous infusion; transfer to 100 mL of an appropriate infusion solution first (0.9% sodium chloride, 5% dextrose, 5% dextrose with 0.9% sodium chloride, or 5% dextrose with 0.225% or 0.45% saline). Do not mix with, or physically add to, other antibacterial drugs; it may be given concomitantly with other antibacterials such as aminoglycosides. Limitations of use: not indicated in patients with meningitis (safety and efficacy not established); to be used only for infections proven or strongly suspected to be caused by bacteria. Seizures have occurred especially when recommended dosages were exceeded - close adherence to the dosing schedule is urged. The indications section was truncated at the source-fetch limit after 1.6 (bone and joint infections); the summary bullet list names all eight indications. No UK SPC for imipenem with cilastatin is listed on eMC — this dose is from US prescribing information (Imipenem and Cilastatin for Injection, USP, WG Critical Care, LLC) and must be verified against UK practice.

Paediatric dose

Route: Intravenous infusion (doses of 500 mg or less over 20 to 30 minutes; doses greater than 500 mg over 40 to 60 minutes)
Frequency: By age: every 6 hours (3 months and over; also 4 weeks to 3 months); every 8 hours (1 to 4 weeks); every 12 hours (under 1 week)
Max: Maximum total daily dosage not to exceed 4 g/day (label: based on studies in adults)
dosePerKg is left null because the label gives an age-banded schedule with a range for the older group. Table 2 (non-CNS infections), as retrieved: 'Greater than or equal to 3 Months of Age: 15-25 mg/kg every 6 hours'; 'Less than or equal to 3 months of age (Greater than or equal to 1,500 g body weight): 4 weeks to 3 months of age 25 mg/kg every 6 hours; 1 to 4 weeks of age 25 mg/kg every 8 hours; Less than 1 week of age 25 mg/kg every 12 hours.' Label restrictions: 'not recommended in pediatric patients with CNS infections because of the risk of seizures'; 'not recommended in pediatric patients <30 kg with renal impairment, as no data are available'. Do not use diluents containing benzyl alcohol to reconstitute for neonates (toxicity associated in neonates). US labelling — verify against a children's formulary.

Dose adjustments

Renal

Adults: dosage reduction is required for creatinine clearance (CLcr) below 90 mL/min (use Cockcroft-Gault on a steady-state serum creatinine). Table 3, imipenem dose by CLcr band (90 or more / 60 to below 90 / 30 to below 60 / 15 to below 30 mL/min): susceptible organism - 500 mg every 6 hours / 400 mg every 6 hours / 300 mg every 6 hours / 200 mg every 6 hours, OR 1000 mg every 8 hours / 500 mg every 6 hours / 500 mg every 8 hours / 500 mg every 12 hours; intermediate susceptibility - 1000 mg every 6 hours / 750 mg every 8 hours / 500 mg every 6 hours / 500 mg every 12 hours. CAUTION: the source table was flattened when fetched; the band assignment above follows the column order of its CLcr header and must be checked against the original DailyMed table before use. CLcr below 15 mL/min: do not give unless haemodialysis is instituted within 48 hours. Haemodialysis: use the dosing for the 15 to below 30 mL/min band, give after the haemodialysis session and at intervals timed from its end; recommended only when benefit outweighs the risk of seizures. Inadequate information for peritoneal dialysis. Adults with CLcr 30 mL/min or less (with or without haemodialysis) had a higher risk of seizure activity: adhere closely to the dosing guidelines and monitor creatinine clearance regularly. Not recommended in paediatric patients under 30 kg with renal impairment. No dosage adjustment is required based on age alone.

Dose independently re-sourced from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Known hypersensitivity to any component of imipenem and cilastatin for injection

Side effects

  • Local: phlebitis/thrombophlebitis (3.1%), pain at injection site (0.7%), erythema at injection site (0.4%), vein induration (0.2%)
  • Gastrointestinal: nausea (2%), diarrhoea (1.8%), vomiting (1.5%)
  • Skin: rash (0.9%), pruritus (0.3%), urticaria (0.2%); fever (0.5%); hypotension (0.4%)
  • Nervous system: seizures (0.4%), dizziness (0.3%), somnolence (0.2%); confusional states and myoclonic activity reported
  • Serious hypersensitivity (anaphylactic) reactions, occasionally fatal; severe reactions have been reported in patients with penicillin hypersensitivity treated with another beta-lactam
  • Clostridioides difficile-associated diarrhoea, from mild diarrhoea to fatal colitis (may occur over two months after treatment)
  • Overgrowth of non-susceptible organisms and development of drug-resistant bacteria with prolonged use
  • Paediatric patients 3 months and older (>1%): diarrhoea, rash, phlebitis, gastroenteritis, vomiting, IV site irritation, urine discoloration; neonates to 3 months (>1%): convulsions, diarrhoea, oliguria/anuria, oral candidiasis, rash, tachycardia

Interactions

  • Ganciclovir - generalised seizures reported; do not co-administer unless the benefit outweighs the risk
  • Probenecid - increases plasma level and half-life of imipenem; concomitant use not recommended
  • Valproic acid / divalproex sodium - reduces valproic acid concentrations below the therapeutic range with risk of breakthrough seizures; concomitant use generally not recommended - consider antibacterials other than carbapenems; increasing the valproate dose may not overcome the interaction
  • Other antibacterial drugs - do not mix or physically add to them (may be given concomitantly, e.g. aminoglycosides)

Monitoring

  • Careful inquiry about previous hypersensitivity to penicillins, cephalosporins, other beta-lactams and other allergens before starting
  • Creatinine clearance, regularly in renal impairment (seizure risk higher at CLcr 30 mL/min or less); in the elderly, monitoring renal function may be useful
  • If focal tremors, myoclonus or seizures occur: evaluate neurologically, start anticonvulsant therapy if not already instituted, and re-examine the imipenem/cilastatin dose; continue anticonvulsants in patients with known seizure disorders
  • Evaluate for Clostridioides difficile-associated diarrhoea if diarrhoea occurs, including after the course has finished
  • Repeated evaluation of the patient's condition for superinfection during prolonged use

Clinical monograph

How it works

Imipenem inhibits bacterial cell-wall synthesis by binding penicillin-binding proteins, while cilastatin inhibits the renal enzyme dehydropeptidase-I to prevent imipenem breakdown in the kidney and raise its urinary concentration.

Prescribing in practice

  • It lowers the seizure threshold, so use with caution in patients with central nervous system disorders, renal impairment or a history of seizures.
  • Cross-reactivity can occur in patients with severe beta-lactam hypersensitivity, and use should follow antimicrobial stewardship to preserve carbapenem activity.
  • Dose adjustment is required in renal impairment; consult the SPC and current prescribing references for regimen and administration.

Monitoring

Monitor renal function to guide dosing, observe for seizures and signs of hypersensitivity, and review for superinfection including Clostridioides difficile during prolonged use.

Counselling the patient

  • Report any twitching, confusion or fits during treatment without delay.
  • Tell the team about any previous serious reaction to penicillins or other antibiotics.
  • Report severe or persistent diarrhoea, which should be assessed before taking any anti-diarrhoeal.

Evidence & guidelines

Carbapenems such as imipenem with cilastatin are reserved for serious or resistant infections under antimicrobial stewardship, consistent with NICE and UK antimicrobial guidance.

Reference: UK AMR guidance; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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