s not Herx reaction.
Metronidazole can act as a neurotoxin. It can cause lesions to
form on the brain, mainly on the cerebellum, but they can also be found on the basal ganglia, midbrain...even the brain stem. This causes a lot of effects, including motor control issues (usually difficulty speaking and walking, but it can affect other things), but psychiatric issues are also high on the list--aka, an "altered mental state.