Let me begin with a passage from the Digest of Roman Law within the section on the Lex Cornelia on murderers and poisoners (D.220.127.116.11):
It is laid down by another decree of the senate that dealers in cosmetics are liable to the penalty of this law (the Lex Cornelia on murderers and poisoners) if they recklessly hand over to anyone hemlock (cicuta), salamander, aconite, pine-worms (pituocampae), or buprestis, mandragora, or, except for the purpose of purification, cantharis beetles.
This particular decree of the senate was preserved by the jurist Marcian (active c. 200 and 222 CE), but the actual decree could date from any time between 81BCE, when it was passed, and Marcian’s own day. The main test of the law was not whether or not a murder had been committed, as it is with most modern legal systems, but the intent to murder. Penalties ranged from relegation (temporary exile) to death by wild animals.
Negligence should not have exposed someone to its penalties, but there is evidence in both the legal and literary record of medical professionals who unwittingly aided in a murder being prosecuted under the Lex Cornelia. Galen, for instance, mentions one unfortunate doctor who was executed when a wicked stepmother (of course) claimed a drug was for her own use, only to have her slaves slip it into her stepson’s food.
Other examples preserved in the Digest involve gynecologists, aphrodisiacs, and abortifacients; gender bias very likely accounts for the departure from the intent-test. Add to that the demographics of medical professionals in the Roman Empire–many of them were slaves and freedmen–and the pattern becomes even more clear. Elite moral panic likely drove the legislative policy in this decree of the senate, with chilling effects. Under it, suppliers are held liable for selling commonly used pharmaceutical ingredients.
So would these highly toxic items that an ancient pharmacist would carry? Absolutely! Dioskourides, author of a first century CE pharmacy manual (and standard reference for Roman pharmacists), listed several uses for them.
We begin at the end with Blister Beetles (Cantharis, buprestis, pituocampae): Here, I am grouping three similar insects, just as Dioskourides did (2.61). These insects are more popularly known as “Spanish Fly.” The oil produced by these beetles causes the skin to blister, and this made it a useful item for removing growths.
But Dioskourides does not mention its most famous application, and most dangerous–blister beetle poisoning irritates the urogenital tract, causing an erection. It was, in essence, ancient Viagra. It seems to have been responsible for quite a few accidental and embarrassing deaths, and likely accounts for the general anxiety surrounding the use of aphrodisiacs in Roman law and armchair scientists like Pliny the Elder. It also shows up in some cringe-worthy gynecological recipes, and must have caused many a woman severe discomfort.
Cosmetics sellers would stock it for people with warts, women would keep it handy, and Roman legislators were concerned. It is hardly surprising that this class of insect dominates the senate’s decree.
Aconite, also known as monkshood, wolfsbane, and the “queen of poisons,” is best known today as Professor Snape’s go-to icebreaker question for Potions class. It is deadly–strong enough to cause numbness when it comes in contact with the skin–and that is precisely what put it on the senate’s list. Dioskourides 4.77 declares it useful for killing wolves, and nothing else, but urban sales were almost certainly aimed at eliminating human nuisances like abusive slaveowners and inconvenient husbands, at least in the minds of lawmakers.
Another alumna of Harry Potter, the mandrake is best known for its use in magic. However, it also has a strong effect as a sedative and anesthetic; it was used as such into the early 1900s. But too much could cause death, as Dioskourides warns in his lengthy list of applications (4.75). It’s hallucinogenic properties, too, combined with its sedative effects, would have made it a prime candidate for abuse and accidental death. No wonder it makes the senate’s list!
Even today, Hemlock remains infamous for its role in the death of Socrates. So why on earth would a pharmacy sell it? Dioscorides 4.78 recommends it for topical applications only, first as a cure for shingles and erysipelas, both common and painful skin conditions. He goes on to prescribe it to stop lactation, to keep youthful breasts small, and, alarmingly, to cause a boy’s testicles to shrivel. The most shocking suggestion, though, is that it be applied to the testicles to prevent nocturnal emissions – surely a recipe for disaster if the man in question failed to wash his hands carefully.
So we have in this list a number of items common in recipes and associated with women and medical professionals, both of whom might respond to their systemic oppression with the covert violence of poisoning. If you were to open a pharmacy in the bustling streets of the Roman empire–especially if you were a woman, freedman, or both– it would be best to think twice about why your patient is so keen to buy his cantharis in bulk.
 The ingredients in cosmetics and pharmacy were often similar, and likewise cosmetics were made to also have medical benefits.
 See Lily Beck’s excellent translation and commentary for the most likely identification of the species involved. Taxonomy and nomenclature in antiquity is imprecise by modern standards; it can be difficult to link ancient names to known species.
 For example, Natural History 25.25: “I do not include abortifacients in my account, and not even love potions, remembering that Lucullus the most famous general perished from such a potion.”