Preventive is the original adjective corresponding to prevent, but preventative has gained ground and is now a common variant. The two share all their definitions.
As of early 2013, preventive is about three times as common as preventative in general web searches. And as the ngram below suggests, preventive has been far more common in published books for the last two centuries at least:
The prevalence of the shorter form is seen throughout the English-speaking world, but the longer form is especially common outside North America. In British news stories from 2012, for instance, the ratio of preventive to preventative was very nearly 1:1, while it was almost 10:1 in U.S. news stories from the same period.
Since publishing this post, we’ve received comments saying that preventive is an adjective and preventative is a noun. This would be a useful distinction, but it is not consistently borne out in practice (the NYT example below notwithstanding). Moreover, we find no English reference sources that make the distinction, and those that mention the issue at all simply recommend preventive over preventative without differentiating them.
Preventive and preventative belong to the troublingly inconsistent class of -tive/tative word pairs that also includes interpretive/interpretative, exploitive/exploitative, authoritive/authoritative, and many others. What makes these pairs so troubling is that they have consistently flouted any rules English authorities have attempted to impose, and there is no consistency in how they are formed. What form becomes preferred is decided by usage, and usage is rarely guided by concerns of logic or consistency.
Examples of preventative are easily found in recent news stories:
Free preventative services coverage kicks in on many health plans this coming September. [Babble.com]
Shouldn’t all fishing come to a halt as a preventative measure? [CNN.com]
We propose that preventative interventions can be developed on college campuses. [Stress and Mental Health of College Students, M.V. Landow]
But in edited publications and published books, the shorter form still prevails. Examples such as these are more common:
Given this context, it has been difficult to initiate preventive interventions for children who are only beginning to develop early symptoms. [Lewis's Child and Adolescent Psychiatry]
And people weren’t getting tests or preventive care that could help them avoid heart attacks, diabetes or cancer. [Wall Street Journal]
Preventive measures are simple and cost-effective. [Financial Times]
Oddly, this New York Times op-ed uses both words within a single paragraph, with preventative used as a noun and preventive as an adjective:
But we cannot allow ourselves to forget for even a moment that force is effective only as a preventative—to prevent the destruction and conquest of Israel, to protect our lives and freedom. Every attempt to use force not as a preventive measure, not in self-defense, but instead as a means of smashing problems and squashing ideas, will lead to more disasters. [New York Times]
This writer obviously thinks of preventive is an adjective and preventative is a noun, a distinction that, again, is not consistently borne out in broader usage.