This member of the mint family has been cultivated for centuries and its merits have been written about by the likes of Pliny, Dioscorides and one of the original botanists, Linnaeus.
There are two varieties of the herb, both an American and English version. Though somewhat different in their appearance, both varieties offer similar benefits.
Like most herbs, pennyroyal’s traditional uses range from easing headaches to curing the common cold and was often cited by Anglo-Saxon and Welsh medical books for its many applications.
Similar to other members of the mint family, the leaves are highly aromatic, though not as sweet smelling as those of peppermint or spearmint. Often, pennyroyal’s leaves are infused into a tea and studies show that the menthol component (pulegone) in the leaves often act as an antihistamine and can help clear nasal and bronchial passages.
Throughout history pennyroyal tea was used to quell digestive troubles, reducing instances of flatulence in particular. It was also applied topically to wounds, cold sores, eczema and other rash-like conditions.
One of its main uses, however, was to aid in female menstruation by helping to induce delayed or irregular cycles. Because of its highly potent and often unpredictable effects on the uterus, pregnant women need to use extreme caution when ingesting the herb as it can easily lead to miscarriage.
Today, the herb needs to be used with care as there have been reported cases of toxicity in high doses. When ingesting pennyroyal tea, special consideration needs to be taken as to the dosage and should only be consumed for five days at a time as it may have a toxic effect on the liver and other internal organs if used for prolonged periods. A professional herbalist can prepare an herbal blend that is safe to consume.
There are, however, other ways in which you (and your dog) can benefit from this complex herb. Dried pennyroyal leaves are a proven flea deterrent. Sprinkle them around your pet’s bedding or crates to keep the pests at bay. A few leaves can also be wrapped 
around Fido’s collar if you are going to be out hiking.
And keep a few stems handy yourself as they will keep the gnats from swarming. You can also prepare an infusion to apply topically or sprinkle around outdoor seating areas to allow for outdoor entertaining without the unwanted guests. And pennyroyal will also keep your garden growing strong as it often deters burrowing insects from attacking your beds.
-The Alternative Daily
Sources:
http://www.motherearthnews.com/natural-health/pennyroyal-safety.aspx#axzz2kS6PECc1
http://www.mskcc.org/cancer-care/herb/pennyroyal
http://www.herbal-treatment-remedies.com/pennyroyal-herb.html
http://www.botanical.com/botanical/mgmh/p/pennyr23.html
http://www.drugs.com/npc/pennyroyal.html
http://www.pfaf.org/user/Plant.aspx?LatinName=Mentha+pulegium