In one study in 6 subjects
phenobarbital caused reductions of 10 to 86% in the AUC of
paroxetine, but the
mean values were unaltered. One subject showed a 56% increase in their AUC. Primidone is metabolised to
phenobarbital and may therefore interact similarly with rifampicin.
Although there seems to be little correlation between
plasma paroxetine levels and its
efficacy, be alert for the need to increase its dosage if
phenobarbital is given. Note that
SSRIs can decrease the seizure threshold.