was based on the fruit of a particular genus of the fennel plant, a flowering herb that only grew along a certain shoreline in modern day Libya. The heart-shaped fruit of the Silphium plant was known to be something of a cure-all, and was used to treat warts, fever, indigestion and a whole host of other ailments. But it was Silphium’s powers as a contraceptive that made it one of the most valuable substances in the Roman world, to the point that the plant appears on several different pieces of ancient Roman currency. Women would drink Silphium juice every few weeks, and this would be enough to prevent pregnancy. Using the herb would even terminate an existing pregnancy if used correctly, which would make Silphium one of the earliest methods of abortion.The Straight Dope adds:
Silphium was one of the most sought after drugs of the ancient world, and its use spread rapidly across Europe and into Asia. But despite its remarkable effects, the particular genus of plant needed would only grow in one area along the Mediterranean in North Africa. Its scarcity, combined with an overwhelming demand, more than likely led to over harvesting, which drove the plant into extinction. Because the particular species no longer exists, modern scientists are unable to examine Silphium to see if its powers of contraception were really as effective as Roman historians and poets would lead one to believe, or if there were any adverse side effects. Still, it is worth noting that other herbs that are chemically similar to Silphium have been proven to have a fairly high rate of preventing pregnancy.
Although silphium is no longer around, modern studies of the closely related plant asafetida show a 50 percent success rate in preventing implantation of fertilized eggs in rats, and it's nearly 100 percent effective when fed to them within three days of mating. Likewise, studies of wild carrot have shown that it blocks production of progesterone, necessary for the uterus lining to maintain the fetus. The seeds of Queen Anne's lace are still used as a birth control method today.This is what Pliny says (Natural History XXII:49):
Laser, a juice which distils from silphium, as we have already stated, and reckoned among the most precious gifts presented to us by Nature, is made use of in numerous medicinal preparations. Employed by itself, it warms and revives persons benumbed with cold, and, taken in drink, it alleviates affections of the sinews. It is given to females in wine, and is used with soft wool as a pessary to promote the menstrual discharge.