Cimetidine, but not oxmetidine, penetrates into the cerebrospinal fluid after a single intravenous dose.

1 Thirty-six patients with various neurological diseases or symptoms received single intravenous doses of either cimetidine 400 mg (n = 19) or oxmetidine 200 mg (n = 17), 15 or 60 min before a diagnostic lumbar puncture. 2 In the 15 min CSF samples concentrations of cimetidine were detectable but not measurable in 5 and non-detectable in 3 patients. 3 In the 60 min CSF samples the concentrations of cimetidine were detectable in all 11 patients and were measurable in 8 of these patients with a mean +/- s.e. mean of 0.12 +/- 0.01 microgram/ml. These CSF concentrations were correlated to simultaneously measured plasma concentrations (P less than 0.01). The mean ratio CSF/plasma concentration was 0.03. 4 No detectable concentrations of oxmetidine were found either in the 15 min (n = 9) or in the 60 min (n = 8) liquor samples. 5 Cimetidine penetrates the blood-drain barrier slowly and not freely after a single dose. Our data suggest that the new histamine H2-receptor antagonist oxmetidine does not cross this barrier.