No Detectable Phenytoin Plasma Levels After Topical Phenytoin Cream Application in Chronic Pain: Inferences for Mechanisms of Action.

Plasma phenytoin levels were below the limit of detection after topical use of phenytoin cream formulations up to 30% on intact skin and mucous membrane for the treatment of chronic pain, without side effects emerging. This finding suggests that the mechanism of analgesic action resides in the skin.