I often hear these words used interchangeably.

Problem:
It was not clear — at least not to me, anyway — which word is correct.

Explanation:
The word “preventive” as an adjective means serving to hinder or prevent.

The word “preventive” as a noun means a preventive measure or agent.

The word “preventive” dates back to 1630-1640.

The word “preventative” is noted by some dictionaries as being a synonym of the word “preventive” — whether as an adjective or as a noun.

However, at least one dictionary — Webster’s Revised Unabridged Dictionary — says that “preventative” is incorrectly used instead of “preventive”.

Solution:
Given these facts, use “preventive” instead of “preventative” — as either an adjective or a noun.