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1650s, from prevent + -ative. OED points out that preventive is better-formed. As a noun, from 1774.
1630s, from Latin praevent-, past participle stem of praevenire (see prevent), + -ive. As a noun, from 1630s; in medical use from 1670s.
preventive pre·ven·tive (prĭ-věn'tĭv) or pre·ven·ta·tive (-tə-tĭv)adj. Preventing or slowing the course of an illness or a disease; prophylactic. n. A preventive agent or treatment.