Analysis of a clinically important interaction between phenytoin and Shankhapushpi, an Ayurvedic preparation.

During the course of routine plasma drug level monitoring an unexpected loss of seizure control and reduction in plasma phenytoin levels was noticed in two patients who were also taking 'Shankhapushpi' (SRC), an Ayurvedic preparation. Therefore, the present study was undertaken in rats to investigate any SRC-phenytoin interaction from both pharmacokinetic (serum levels) and pharmacodynamic (electroshock seizure prevention) aspects. Single dose SRC and phenytoin (oral/i.p.) coadministration did not have any effect on plasma phenytoin levels but decreased the antiepileptic activity of phenytoin significantly. On multiple-dose coadministration, SRC reduced not only the antiepileptic activity of phenytoin but also lowered plasma phenytoin levels. SRC itself showed significant antiepileptic activity compared to placebo and is worth further investigation. However, the clinical combination of SRC with phenytoin is not advised.