Andrea Salinas for years used lavender-scented deodorant, figuring the “all-natural” product was healthier than commercial brands. She also used lavender oil as a body spray.

Recently, Ms. Salinas, a clinical social worker in San Francisco, started breaking out with a red, itchy rash that started in her underarm area and spread across her torso. Tests by a dermatologist turned up an unexpected culprit: She had developed an allergy to lavender.

The 44-year-old switched to a commercially available deodorant. “I love the smell of lavender,” she says, but now “there’s none whatsoever” in her house.

Health-conscious consumers increasingly have been snapping up lotions, soaps and other personal-care products drawn from plants and other biological sources. Some people also are drawn to marketing pitches that the products aren’t tested on animals. Sales in the U.S. of high-end, facial skin-care products marketed as “natural” grew more than 25% between 2013 and 2015, according to market-research firm NPD Group Inc. In other places, including in Europe, natural is a common marketing pitch among many leading brands.

But for some people, the natural substances can set off allergic contact dermatitis, with symptoms of rash and occasionally blisters. Some dermatologists who specialize in the condition say they are seeing more cases tied to ingredients such as lavender, peppermint and jasmine. “These ingredients are ubiquitous,” says Nina Botto, assistant professor of dermatology at University of California, San Francisco. “Everybody is starting to see more of this.”