"...[mandragora is] given for injuries inflicted by serpents and before incisions or punctures are made in the body, in order to insure insensibility to pain. Indeed for this last purpose, for some persons the odour is quite sufficient to induce sleep."
Thus Roman author Pliny in Historia Naturalis (c.77AD) describes what would later be known as inhalation anaesthesia.
Refs and Further Reading
Anaesthesia and Anaesthetics