PHENOBARBITAL AND RIFAMPICIN (RIFAMPIN)

Phenobarbital possibly modestly increases the clearance of rifampicin (up to a 40% reduction in rifampicin levels in one study). The effect of rifampicin on phenobarbital levels is unknown, but note that rifampicin markedly increases the clearance of hexobarbital. Primidone is metabolised to phenobarbital and may therefore interact similarly with rifampicin.
The outcome of concurrent use is unclear. Concurrent use need not be avoided, but be alert for a reduced response to both drugs.