If you mean dangerous in terms of "effective", then there is decent evidence of widespread knowledge amongst certain groups (midwives and difficult-to-document "wise women", prostitutes) in the later Middle Ages (so, let's say after about 1300) of how to procure a fairly late-term abortion through the use of drugs. I don't see why the same couldn't have been true at an earlier point. It may have been the case that an oral culture of abortifacient birth control existed amongst women (John Riddle argues this), basically for family planning reasons, but the evidence is hard to come by.

The archival evidence I've looked at (from the context of prostitution, fifteenth century, southern Germany) shows women demonstrating knowledge of abortifacient drinks where the key ingredients seem to have been cloves, Queen Anne's Lace, and periwinkle, with strong alcohol as a mixing agent. Purely on anecdotal evidence, this seems to have been very effective (I have a yet-to-be-published case from 1471 of a woman aborting at 24 weeks, though not sure how they managed to verify the age of the foetus).

Stuff to read:

John Riddle, Eve's Herbs

Wolfgang P. Müller, The Criminalization of Abortion in the West: Its Origins in Medieval Law

Hope that helps!

As an additional question, how effective were they?