Why is caution advised when Avonex (interferon beta-1a) is combined with certain antiepileptics or antidepressants?

Caution is advised when Avonex (interferon beta-1a) is combined with antiepileptics or antidepressants that have a narrow therapeutic index and depend largely on hepatic cytochrome P450 for clearance, because interferons have been reported to reduce these enzymes’ activity.