I've seen claims that it was a highly effective contraceptive with no side effects, yet we aren't totally sure what it actually was. So what hard evidence do we have about its effects on the human body? Can we be sure its traits haven't been exaggerated, or that its side effects didn't go unnoticed or ignored?
We've actually got very little testimony of silphium as a contraceptive. It was primarily used as a spice.
The problem is that silphium appears to have become extinct shortly after the Romans took control of Cyrene. Cyrene is the only place it grew. Prior to that, silphium had been an important mainstay of the Cyrenaic economy -- here you can see it as a symbol of the city, on one of their coins -- but Pliny the Elder tells us that enormous quantities were hoarded in Rome in the time of Caesar, and that by his own time about a century later it was no possible to get hold of it. He tells an anecdote of a single plant being found and sent to the emperor. Basically, Roman use of silphium destroyed the specieis in a matter of decades.
The references we have to silphium being used as an contraceptive or abortifacient date to Pliny's time or later -- that is, the plant was already extinct by the time people were writing about its use as an abortifacient. That makes it hard to put much stock in what they do say (which is still very little).
So no, there's no basis for calling it a 'highly effective' or 'with no side effects'. We don't have that kind of information. But yes, these post-extinction sources do attest to a reputation for being used as a contraceptive and abortifacient.
The plant itself is described by Pliny, Natural history 19.38-45, and Dioskourides, De materia medica 3.80 (PDF translation here). Here are the chief features:
The stalk could be cooked and eaten, but it was apparently most valued for the juice that could be harvested by making incisions in either the stalk or, preferably, the tuber. This was called 'Cyrenaic juice'.
The best juice was reddish (hypérythros) and translucent, according to Dioskourides. It was pungent but not foul-smelling, and had potent effects on trying it for the first time: Dioskourides reports that it made you break out in a sweat all over, but that it didn't make your mouth smell bad afterwards; and Pliny says it was a purgative for the first six weeks of consuming it, and when cattle were fed it, after they got used to it 'the beasts grew fat and produced meat of splendid quality'. For trade, the juice was preserved by mixing it with bran or beanflour. It seems to be in that form that it was used as a garnish on various foods.
The extant sources do talk of alternatives that grew in other parts of the world, some of which still survive -- Syrian magydaris, which is asafoetida, and Persian (Persian asafoetida), Median, and Armenian plants. These varieties of fennel were all that was available in Pliny's time, and were used in place of silphium, but the ancient reports are consistent in declaring that they were all inferior to silphium, and made your breath smelly.
For the medical use of silphium as an emmenagogic (inducing menstruation) and abortifacient, we rely on two principal sources: Dioskourides, and the 2nd century medical writer Soranus.
Dioskourides reports a wide range of supposed medical uses: dispersing incipient cataracts; mouthwash and treating toothache; treating poisonous animal and scorpion bites, and poisoned arrows; sore throats, and so on, and so on. Dioskourides gives this kind of list for all the plants and other foods that he discusses, and it's best not to put much stock in them. Towards the end of his list of supposed effects, he says,
Drunk with pepper and myrrh, it brings on the menstrual period.
It'd be best to take all of this with a pinch of salt, except that Soranus corroborates this bit in a list of contraceptives, and Soranus' list is much more plausible than Dioskourides. Soranus prescribes four recipes to induce menstruation, which therefore double up as contraceptives and early-stage abortifacients:
John Riddle's Contraception and abortion from the ancient world to the renaissance (1992) comments at pages 29-30 that, of the plants in these recipes, most have been found in modern studies to have some effect as contraceptives and emmenagogics, particularly rue. Rocket and cow parsnip haven't been studied for this effect, and we can't study silphium itself. Riddle judges that these recipes are therefore likely to have been effective to some extent.
But ancient medical writers come up with the most ridiculous nonsense when they're talking about preventing pregnancy: things like getting the possibly-pregnant person to do seven jumps with the feet hitting the buttocks to make the semen fall out, that kind of thing. There's loads of good material for /r/badwomensanatomy. This tells us that they weren't basing their judgements on well managed blind studies or anything like that: it was all hearsay and anecdata. The fact that some of Soranus' recipes may have have some effect doesn't tell us anything about their reliability in modern statistical terms.
It'd be unwise to take medical advice from an ancient medical writer. When plants do have an effect as a contraceptive or abortifacient, it's generally because they're really very bad for you. Dioskourides' talk of silphium making your whole body break into a sweat doesn't sound to me so much like a pleasant spice, but more like mild poisoning. Rue isn't something you should be consuming unless you really know what you're doing.
Probably the most commonly cited abortifacient/emmenagogic plants referred to in ancient Mediterranean texts -- for there are many, many more than just silphium -- is pennyroyal. It's even joked about in that capacity by Aristophanes, who jokes about a woman's genitals by purportedly talking about the region she comes from (Lysistrata 88-89) --
And by Zeus,
what a well tended country, weeded well, and planted with pennyroyal!
indicating that she's ready for sex because she's had her pubes shaved and she's on contraceptives. There's a similar joke in the Peace (710-712: 'But won't it do damage if I plough (the) Harvest [the name of a girl]?' 'Not if you add some pennyroyal into the mix').
And pennyroyal genuinely is an effective way to end a pregnancy. It's just that the side effects may include death. As little as 5 grams, in one report I've seen, can be seriously toxic. Ancient contraceptives are dangerous.
There's heaps more to this than just silphium: Riddle's book is well worth a read if you can get hold of it.
Edit: there's an amazing article by Sarah Nelson, 'Persephone's seeds: abortifacients and contraceptives in ancient Greek medicine and their recent reappraisal' (2009) which I also encourage you to take a look at! She looks at specific modern studies of the effects or pennyroyal, rue, and Ferula (which almost certainly included silphium), and how they have their effect. She notes that extracts of modern Ferula asafoetida resulted in a 67% reduction in pregnancies in rats after 10 days, and a greatly reduced litter size (average 7.5 per rat in the control group, 1.2 per rat in the treatment group); but the mode of action was unclear. No tests had been done on humans at the time of Nelson's article.
