(1) Locusta, a woman of ancient Rome, who was employed by the Empress Agrippina to poison her husband Claudius. Nero employed the same woman to poison Britannicus and others.
(2) The Borgias (Pope Alexander VI. and his children, Caesar and Lucrezia) were noted poisoners.
(3) Hieronyma Spara and Toffania, of Italy. (See Aqua Tofana.)
(4) Marquise de Brinvilliers, a young profligate French woman, taught the art by an officer named Sainte Croix, who learnt it in Italy. (See World of Wonders, part vii. p. 203.)
(5) Lavoisin and Lavigoreux, French midwives and fortune-tellers.
(6) Anna Maria Zweinziger, sentenced to death in 1811.
In English history we have a few instances: e.g. Sir Thomas Overbury was so murdered by the Countess of Somerset. King James, it has been said, was a victim to similar poisoning, by Villiers, Duke of Buckingham.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894