It contains no toxic substance. For some cats it may even be an appetite stimulant. Marijuana intoxication in a cat is a different story however.
Cats can eat catnip. In small quantities or with healthy cats the result is the opposite of what one would expect of catnip. Some people treat nausea or headaches with catnip tea. Cats can ingest catnip and it may even be helpful for their digestive tract.