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Phenobarbital is well tolerated and effective for controlling agitation or preventing convulsion at the end of life. No information is available concerning parenteral bioavailability of phenobarbital when induration develops at the injection or infusion site. We investigated whether induration at injection or infusion site is related to phenobarbital bioavailability via parenteral routes of continuous subcutaneous infusion and intermittent subcutaneous or intramuscular injection.A retrospective analysis was conducted on the medical data obtained from 18 patients who received chronic subcutaneous or intramuscular injections of phenobarbital for the prevention of convulsions and underwent plasma concentration monitoring of the drug. Patients whose concomitant medications were altered during the observation periods were excluded from the analysis. Comparisons were performed for concentration/dose (C/D) ratios obtained from patients with induration at injection or infusion sites (induration group, n = 6) and those without induration (noninduration group, n = 12). P < 0.05 was considered statistically significant.The induration group showed significantly reduced C/D ratio compared with the noninduration group [median (range): 0.131 (0.114–0.334) versus 0.219 (0.180–0.322) d/L, P < 0.05). Assuming that systemic clearance was constant in our patients, changes in the C/D ratio would have contributed to 40% (median) reduction in bioavailability of the drug from the injection or infusion site.Our data suggest that absolute bioavailability of phenobarbital may be reduced when induration develops at the injection or infusion site in patients treated parenterally by continuous subcutaneous infusion or intramuscular injection.