Phenobarbital decreases the levels of
felodipine (bioavailability reduced by more than 90%),
nifedipine (AUC reduced by 60%) and
verapamil (bioavailability reduced 5-fold). Other
calcium-channel blockers are expected to interact similarly with
phenobarbital. Note that
primidone is metabolised to
phenobarbital and therefore may also interact.
Monitor the outcome of concurrent use, being aware that the dose of calcium- channel blocker may need to be raised. Given the size of the reductions seen it may be prudent to consider alternatives.