According to Merriam-Webster Online, preventive and preventative (as  in preventive or preventative medicine) are interchangeable. Both words originate in the 1600s, though preventive precedes preventative by about 40 years.

My preferred word of choice? Preventive: it has one less syllable and rolls off the tongue more easily.

See also: Merriam-Webster Dictionary’s entries for preventive and preventative.