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It's safe but I'm not sure why you would want to. First of all, Febreze works by wetting a surface, where it molecularly surrounds odor particles, which evaporates away as the surface dries. Putting Febreze in an oil warmer does none of this since no surface or fabric gets damp. Secondly, Febreze fabric refresher is water based. That means it'll evaporate quickly so the container will be empty within days. Oil, on the other hand, doesn't evaporate unless warmed. And even then, it evaporates slowly. That's why scents are dissolved in oils , not water.