Phenobarbital Alkaline pH Incompatibility — SCE Palliative Medicine MCQ

Correct answer: D — Phenobarbital has a very high pH (10–11) making it chemically incompatible with most other drugs in solution, causing precipitation and degradation

Explanation lettering: E = shown as A · C = shown as B · A = shown as C · B = shown as D · D = shown as E

Phenobarbital has a very high alkaline pH of 10–11 in its undiluted formulation, which causes chemical incompatibility with virtually all other drugs commonly used in CSCI (opioids, benzodiazepines, antiemetics, antipsychotics). Mixing phenobarbital with other drugs in a single syringe results in precipitation and degradation of one or both drugs, reducing efficacy and risking therapeutic failure. For this reason, UK palliative care guidance (Right Decisions—NHS Scotland) recommends phenobarbital be administered via a separate CSCI unless specialist pharmacy compatibility advice confirms safety. Distractor A contradicts the incompatibility. C is incorrect because IV phenobarbital is diluted with WFI, not normal saline, and WFI is standard for CSCI diluents. D is incorrect because, although phenobarbital's first doses may be given IM/IV, ongoing management is via CSCI (subcutaneous infusion). E confuses the issue of solubility with concentration/volume.

Reference: Right Decisions—Scottish Palliative Care Guidelines: Phenobarbital (phenobarbitone). https://www.rightdecisions.scot.nhs.uk/scottish-palliative-care-guidelines/medicines-information/phenobarbital-phenobarbitone/