Measurement of thioridazine in blood and urine.

1 Thioridazine can be specifically, simply, and reliably measured in plasma and urine by gas chromatography using hexane extraction and prochlorperazine as internal standard; fluorimetry is non‐specific. 2 The method can also measure thioridazine ring sulphoxide, and mesoridazine‐plus‐sulphoridazine (M/S). 3 After single doses plasma sometimes shows M/S in addition to thioridazine itself; it always does so on continued treatment. There is great individual variation in both components, and evidence of changes in metabolism during the early weeks. 4 Urinary excretion may be influenced by pH, but between pH 6.0‐ 7.0 about 1% of the daily dose appears in 24 h urine as the following: free thioridazine in microng quantities, M/S and ring sulphoxide each in mg amounts. 5 Patients attain steady state conditions, although plasma levels rise considerably after each dose and settle again in about 10 h. After chronic treatment is stopped to half‐life is at about 30 h. 6 Plasma levels cannot be related to therapeutic response when this is slow, as in schizophrenia, but interpretations are complicated by the production of clinically active metabolites, and by plasma protein binding.