Chloramphenicol Injection and Phenobarbital: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenobarbital
Chloramphenicol Injection
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Phenobarbital toxicity or decreased chloramphenicol effectiveness
Interaction Deep Dive
In general, concurrent use of these drugs results in decreased chloramphenicol concentration by 30 to 40% and an increase of phenobarbital concentration by as much as 40 to 50% as compared to when either drug is used alone12345.
Why it happens (mechanism)
Altered metabolism
Common questions
Can I take Chloramphenicol Injection and Phenobarbital together?
Phenobarbital toxicity or decreased chloramphenicol effectiveness Always confirm with your pharmacist or prescriber before making any change.
How serious is the Chloramphenicol Injection and Phenobarbital interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
Questions for your pharmacist
- Does my dose of Chloramphenicol Injection or Phenobarbital need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (5)
- Kearns GL, Bocchini JA Jr, Brown RD, et al: Absence of a pharmacokinetic interaction between chloramphenicol and acetaminophen in children. J Pediatr 1985; 107:134-139. PubMed
- Windorfer A Jr & Pringsheim W: Studies on the concentrations of chloramphenicol in the serum and cerebrospinal fluid of neonates, infants, and small children. Reciprocal reactions between chloramphenicol, penicillin and phenobarbitone. Eur J Pediatr 1977; 124:129-138. PubMed
- Krasinski K, Kusmiesz H, & Nelson JD: Pharmacologic interactions among chloramphenicol, phenytoin and phenobarbital. Pediatr Infect Dis 1982; 1:232-235. DOI
- Koup JR, Gibaldi M, McNamara P, et al: Interaction of chloramphenicol with phenytoin and phenobarbital: case report. Clin Pharmacol Ther 1978; 24:571-575. PubMed
- Perucca E: Pharmacokinetic interactions with antiepileptic drugs. Clin Pharmacokinet 1982; 7:57-84. DOI
Keep reading about Phenobarbital
Keep reading about Chloramphenicol Injection
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