The woman should wash her mouth after dinner with very good wine. Then she ought to dry [her teeth] well and wipe [them] with a new white cloth. Finally, let her chew each day fennel or lovage or parsley, which is better to chew because it gives off a good smell and cleans good gums and makes the teeth very white." After reading through several recipes of truly awful things for your teeth (sulphuric acid, abrasives like pumice and marble, &c. &c.) it was refreshing to come across this one. Our ancestors who followed these instructions probably did have better teeth than their neighbors. Unlike the highly abrasive dentifrices, which wear down the tooth enamel, and the sugary preparations which lead to cavities, the ingredients here are not only benign but actually helpful to oral health.
"Very good wine" (in addition to being an excellent after-dinner treat, so long as you don't over-indulge) contains alcohol just like our modern mouth washes; alcohol kills the bacteria that cause bad breath. Cloth would be soft enough to not damage the teeth, though it wouldn't reach in between the teeth unless you plucked out a single strand. (Just use dental floss; we've found natural fibers break easily when used to floss. And yes, we've tried.)
As to the herbs listed: parsley is famous for improving bad breath. Fennel wouldn't be so bad either; it is a cooking herb that tastes like anise or licorice, and is used in herbal medicine for digestive complaints. Lovage, while not used as often in herbal medicine, is a cooking herb that tastes like celery. We're not sure it would do your teeth much good, but it probably wouldn't do them harm either. (Of the three choices, we'd pick parsley first-- just as the 12th-century author does.)
To be honest, we were quite surprised to find this little gem of a recipe among all the more harmful ones!
Sources: The Trotula, 12th century; Desk Reference to Nature's Medicine by Steven Foster and Rebecca L. Johnson, 2006.