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

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Abstract
Patients (36) with various neurological diseases or symptoms received single i.v. doses of cimetidine 400 mg (n = 19) or oxmetidine 200 mg (n = 17), 15 or 60 min before a diagnostic lumbar puncture. In the 15-min CSF samples, concentrations of cimetidine were detectable but not measurable in 5 and non detectable in 3 patients. 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 .+-. SE of 0.12 .+-. 0.01 .mu.g/ml. These CSF concentrations were correlated to simultaneously measured plasma concentrations (P < 0.01). The mean ratio CSF/plasma concentration was 0.03. No detectable concentrations of oxmetidine were found either in the 15-min (n = 9) or the 60-min (n = 8) liquor samples. Cimetidine penetrates the blood-brain barrier slowly and not freely after a single dose. The new histamine H2-receptor antagonist oxmetidine apparently does not cross this barrier.