Locusta of Gaul was involved in the poisoning of Claudius on behalf of Agrippina, and of Claudius' son Britannicus on behalf of Nero. Read more about her here. In the ancient world poison (pharmaka) was often associated with witchcraft.
See also Magic, Witchcraft and Ghosts in the Greek and Roman Worlds by Daniel Ogden; also Magic and Magicians in the Greco-Roman World by Matthew Dickie and Magic in the Ancient World (Revealing Antiquity) by F Graf.
Read more about Magic in the Roman World at http://www.theromanforum.com/?p=3874
also at this site:-