From: sheresa

Do-It-Yourself Flea Collar

Find a durable and comfortable collar- a leather strap or heavy but
smooth twine- soak it in pennyroyal oil for twenty-four hours. Then tie
it around your pet's neck and the fleas will flee.
Caution: Do not use pennyroyal if you or your pet are pregnant. In rare
instances, it has caused cats to abort.

Don't use it if you are pregnant either. It is an abortive for folks too. "Uncle B"

"Live and Be Well" by Joan Wilen and Lydia Wilen