APREPITANT AND PHENOBARBITAL

Rifampicin, a potent inducer of CYP3A4, reduces the AUC of aprepitant (and probably its prodrug fosaprepitant) by 91%; reduced efficacy would be expected. Phenobarbital (and therefore probably primidone), another potent inducer of CYP3A4, is predicted to interact similarly.
The UK manufacturer recommends that concurrent use should be avoided.