It has come to my attention that there is some vaguity (my favorite made-up word) in discerning “preventive” from “preventative.” I’ve done some work toward clearing this up in my own mind, and I may as well post the results of my contemplation.
Most online sources are completely useless in differentiating between the two.
That said, and upon deciding there is considerable latitude in reaching a conclusion, there were elements that influenced my personal decision. The International Journal of Epidemiology addressed this confusion in their Letter to the Editor section. They note, as I found, that the Oxford English Dictionary applies the same definition to each word and each entry refers to the other, unhelpfully, but they do suggest that “preventive” is the preferred usage. A retired English teacher in Chicago notes, in his blog, that the two words mean the same but some grammarians prefer “the shorter version” simply because it is shorter.
On the contrary, Wiktionary insists that the two words are “in all senses synonymous” but concedes that some people prefer preventive as an adjective and preventative as a noun. What does that mean? Exempli gratia: “Vitamin C has a preventive effect upon scurvy; vitamin C is a preventative.” Tiscali supports this inference, but Tiscali is a European telecommunications company that offers a reference library on their site (as well as news, online games, and broadband accounts for the UK); they are not a dedicated dictionary or authoritative etymological site. I can’t find what resource they’re pulling their information from.
I’m inclined to adopt that treatment: preventive as adj. and preventative as noun. What would “preventative medicine” become? The design of medical equipment, or actual flu shots for medical equipment? There wouldn’t be any “preventative medicine” because that’s an adjective. But could I enforce this usage across the board sufficiently to impress it in the minds of copywriters and the intended audience alike? Are there enough instances of preventative by itself, as a noun, for people to pick up on this? I don’t know if I’ve ever seen it cross my desk. So, for all intents and purposes, it will appear as though I’ve made the sweeping change to universally impose preventive across the board, with an exception that never arises.