According to the etymology of the word, a philtre (or philter) is a love potion. It is frequently used to describe any type of potion with magickal properties, however the origins of the word from French and Latin show that technically, a philtre specifically refers to potions used for the purposes of love.
Although their popularity in the 1800's onward has declined in favor of poppets, spells and charms, philtres are still in use today. A traditional philtre was usually an infusion of herbs, often psychoactive, in wine or water. Some of them require more obscure or even grotesque ingredients, taste bitter and have an offensive odor. Obviously they can be difficult to administer discretely, hence their frequent replacement by the above measures.
Because of the nature of love spells, most (but not all) Wiccans, do not utilize philtres - feeling that love magick is manipulative or against the Rede. This judgement is left entirely to the individual to decide.
Many of the ingredients of philtres may have physical effectiveness as well as magickal (isn't that usually the case?). Philtres containing alcohol may relax inhibitions, and ingredients such as saw palmetto may improve prostate health. The bark of the Yohimbe tree (Corynanthe yohimbe, of the ruiaceae family) has been used as an ingredient in philtres. Some of the chemical constituents of this herb (such as yohimbine hydrochloride) seem to increase the flow of blood to the penis, countering the effects of certain hormone imbalances that cause blood vessels to constrict with age.
A study published in the Journal of Urology tested the effects of a yohimbe component. After a month, 81% of the men who had experienced this sexual disfunction for under two years showed improvement. There is also some evidence that yohimbe increases adrenaline to nerve endings and increases the production of norepinephrine. These stimulating effects can easily be associated with feelings of love in both men and women.
Some philtres are not used internally, but are used as fragrances. You will find more information on the use of fragrance as an aphrodisiac in the PaganPath Library articles. There are also philtres that require you to rub them on your hands and then touch the person you desire.
Recipes for philtres are numerous, from a simple combination of rose water and spices, to a drop of menstrual blood in a glass of wine. Creating your own with knowledge of herbalism, aromatherapy, and magick will produce the best results.