A bit more insight on pennyroyal:
 
An aside - interesting in the last FML that Sukie and I appear to disagree
on the safety of pennyroyal.  As I indicate that it can be used safely,
her Herbal PDR research suggests hepatotoxicity and the possibility of
abortion.
 
Well actually, both answers are correct, but more explanation is in order.
 
Pennyroyal is a substance that has been used for thousands of years as an
herbal flea repellent.  Derived from the leaves of the pennyroyal plant,
the active ingredient is pulegone - a word derived from the Latin word
"pulex" meaning flea.
 
The key to the toxicity of pennyroyal (I'm not talking about efficacy,
only toxicity), is like most things - how much you give.  The one dog wth
reported pennyroyal toxicosis in the literature received a dose of 60ml of
a preparation of 100% pennyroyal topically, (the owner had picked it up at
a health food store.  This is equivalent to approximately 2000 mg/kg of
pennyroyal topically.  Pennyroyal oil when ingested by humans is toxic at
a dose of 500 mg total (but that's another species entirely, and it was
ingested.) Also, it is likely that an animal doused with the stuff may
ingest part of it.  Pennyroyal oil is used as an abortofacient in humans,
and is generally taken orally.
 
There are no animal studies on the toxicity of pennyroyal available.
 
Now why would I say that the material is generally safe?  Actually, it is
a common compound in many herbal flea remedies and has been for years.
In such remedies, it is present only in very small amounts, not enough to
cause toxicity (and is usually mixed with small amounts of other herbal
remedies.) However, when used straight, or if the commercially available
flea compound is used in high enough doses, toxicity may result.  Plus,
most formulations are available for dogs, so caveat emptor when crossing
species lines.
 
Lest anyone think that pennyroyal is the only flea product that may be
toxic, the answer is that ALL of them, given in high enough doses, may be
toxic.  We employ toxic products to kill fleas - the trick is not to give
it in sufficient amounts to cause toxicity in the animal, too.
Organophosphates and carbamates, though rarely used today are common
examples of toxicants used in flea shampoo.  I remember as a young man,
working as a groomer in a vet's office, that I would douse the animals with
malathion (!!!) not knowing that this drug could lead to marked toxicity to
both the animal and the young groomer!  Today's products, like lufeneron
and pyrethrin have much higher levels of safety, which is increased by the
fact that a single doses are prepackaged, and it takes a real dummy to
cause toxicity.
 
I have a question for anyone considering the use of pennyroyal for flea
control in ferrets or other pets - WHY?  There are far more effective
products available through your vet.  Toxicity is a serious issue, but
efficacy is also important.  Chances are, at safe dosages, these products
are of little use in flea control.  Fleas really don't care much about
odors.
 
With kindest regards,
Bruce Williams, dVM
[Posted in FML issue 3261]