- Concurrent or recent use (≤ 7 days prior to ZD1839 administration) of phenytoin, carbamazepine, barbiturates, rifampicin, oxcarbazepine, rifapentine, modafinil, or St. John's Wort; Note: Because these drugs induce CYP3A4 enzymes and can cause reductions in ZD1839 plasma concentrations below levels thought to be biologically active, patients with concurrent or recent use of these drugs are excluded from the study