Concomitant use of phenytoin, carbamazepine, barbiturates, rifampicin, phenobarbital, or St John's Wort; these drugs induce CYP3A and may decrease levels of taxanes. 5-FU is a strong CYP2C9 inducer, and concomitant use with carvedilol, celocoxib, fosphenytoin, fluoxetine, phenytoin, warfarin and other CYP2C9 substrates should be used with caution.