Serum trihexyphenidyl levels in the treatment of torsion dystonia.

Using a radioreceptor technique, we assayed serum trihexyphenidyl levels in patients with dystonia being treated chronically with high dosage. We found a significant correlation between total daily dose and the daily lowest (trough) serum levels. There was no relationship between serum levels and therapeutic response or toxicity. Toxicity was more closely related to patient age than to serum level. Although levels may be useful to monitor patient compliance, they cannot be used to judge adequacy of therapy.