You've likely seen — or smelled — essential oils before: Small bottles with potent scents, usually sold in stores surrounded by other "natural," "holistic" products.

Essential oils are fragrant plant extracts, made by steaming or pressing plants, according to Johns Hopkins Medicine. They're often used for aromatherapy, which a centuries-old practice of inhaling these oils or absorbing them through the skin with the goal of improving certain health ailments. There are dozens of types of essential oils, including lavender, tea tree, peppermint and lemon oils. Some celebrities, like Bella Hadid, swear by them.

If you're considering getting into the essential oil game, this is what medical experts want you to know first.

How to use essential oils

Essential oils are likely safe to inhale, University of Maryland School of Pharmacy assistant professor Lauren Hynicka, PharmD, BCPS, tells USA TODAY. You can add a few drops to a diffuser, cotton ball or nasal inhaler. If you're going to use them topically, make sure to dilute them in coconut or jojoba oil first.

And make sure you're investing in a high-quality essential oil — Johns Hopkins warns that some companies will dub their products "therapeutic-grade," but that's an unregulated marketing term, not a signifier that it's a product a medical expert would recommend.

What is the number one essential oil for anxiety?

Some research has shown that essential oils can offer some benefit for some health concerns. Lavender essential oil may be beneficial for anxiety, depression and sleep.

Experts caution that there's still a lot unknown about how essential oils work, because most of the studies conducted aren't the highest quality.

"Conducting high quality research with essential oils can be challenging," Hynicka says. She references double-blind studies, during which neither the study subject nor the researcher knows if a placebo or actual treatment is being used until the end to prevent bias.

But as Hynicka points out, it's tough to fake a placebo for essential oils: "Either you smell an essential oil, or you don’t."

Johns Hopkins called some lab studies "promising," but said clinical trials actually using humans were "mixed," with some showing benefits and others showing no improvement in symptoms.

More on essential oilsWhat oils to use, how to use them and safety tips

When should you not use essential oils?

Those who are pregnant, nursing, taking medication and/or have a history of seizures should be wary of using essential oils, Hynicka says. Even if you're not, she recommends taking stock of what ailment you're hoping to solve by using essential oils — could it be better helped with a different form of treatment?

"I would recommend anyone using essential oils mention the reason and how they plan to use essential oils with their doctor or medical provider," Hynicka says, adding that they should be kept away from children and pets.

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