A simple smell evokes so much. Your husband’s cologne. Freshly baked cookies. Newly mowed grass under the warm summer sun. Smells make us remember, make us feel calmer and even make us smile.

Scent is a chemical signature that makes its way through your olfactory system and triggers a response in your brain. That much is clear. But what some people debate is whether a scent can actually affect your physiology—soothing an aching head or calming a nauseated stomach.

Devotees of aromatherapy claim scents such as lavender and orange oil affect them physically, and a new cloud of essential oil products has drifted in to let people experiment with these powerful fragrances using air diffusers and topical applicators.

But as newbies conduct these at-home scent experiments, both aromatherapy experts and detractors worry about those who approach it uninformed. Aromatherapy isn’t safe for everyone, they say, and an essential oil should certainly never be used to replace a medical doctor.

According to the University of Maryland Medical Center website, the practice of using essential oils for therapeutic purposes dates back at least 6,000 years, with the ancient Chinese, Indians, Egyptians, Greeks and Romans also using them in cosmetics and for hygienic and ritualistic purposes.

Then in the 1920s, French chemist René-Maurice Gattefossé applied lavender oil to a burn on his hand and found it soothed and helped heal the wound. Gattefossé went on to research the effects of essential oils on burns, skin infections, gangrene and wounds afflicting soldiers during World War I, and founded what is known as the science of aromatherapy in 1928, according to the UMMC website.

While researchers have studied the affects of aromatherapy for quite some time and it’s been used by professionals from massage therapists to nurses to doctors, it was just three decades ago that aromatherapy had a resurgence, says Valerie Cooksley, a registerd nurse and nurse-aromatherapist who has studied botanical medicine for 30 years.

“I personally saw aromatherapy enter mainstream via pleasure and stress relief in the early 1980s,” says Cooksley, who holds seven aromatherapy certifications, including the International Training Program in Essential Oils at Purdue University.

The appeal has only grown since then, and now a huge new trend in holistic health has fueled a burgeoning mass market for essential oil sales. People are looking for more natural solutions to insomnia and stress, and they’re turning to essential oil products from companies such as Young Living, which exceeded $1 billion in sales in 2015, according to direct sales website BusinessForHome.org. Young Living’s message and products are spread by “distributors,” average people—usually women—who sell the oils direct as one-person businesses. Another company in the game is doTerra, which boasts 1 million such sales reps around the country.

So essential oil diffusers are common on kitchen counters, but they’re popping up in other places, too. Cooksley says more hospitals are using aromatherapy as part of their complementary and alternative medicine offerings, and encouraging and sponsoring their nurses to get certified.

“Many nurses, and some physicians, are trying to catch up to the wealth of information and effective healing tools available to assist with health and well-being,” she says. “Aromatherapy, which is under the umbrella of botanical medicine, is filling the gap where high-tech medicine and lab-created drugs basically cannot.”

Peek into medicine cabinets, behind shower curtains and under sinks, and you’ll notice many Americans have replaced traditional synthetic chemical products with natural-ingredient and homeopathic remedies, shampoos, lotions and cleaning products. This attitude of ridding one’s body and home of manmade chemicals fits in nicely with essential oil use.

Jeanne Bello, a married mother of two, has not only converted all her mainstream home products to natural ones, she actually makes most of them herself, including lip balm, lotion, cleaning solution and citronella candles. The latter “smell way better than the ones at the store,” she says.

“I realized that most of these things I’d buy at the store had ingredients I couldn’t pronounce,” says Bello, who lives in Channahon, Ill. “I’d rather make something from ingredients in my cabinet. So I decided to free my home from chemicals. It’s a greener way to live.”

Each of her recipes includes one or more essential oils. Bello is a distributor for Young Living. She became one after becoming engrossed in the world of essential oils. A friend got her interested, and before long she plunged into online research and participation in an aromatherapy Facebook group.

Now she uses essential oils every day, in almost every aspect of her life. She puts a few drops of lemon oil in her water every morning for energy, and diffuses an oil blend in her bedroom each night for more restful sleep. After an upsetting first visit to the dentist for her now 5-year-old son, Ethan, Bello decided to apply Young Living’s Stress Away oil blend to the back of his neck in hopes it would help him stay calm for the next visit.

“He was like a different kid,” she says. “He wanted to go first, and he was excited to be at the dentist.”

Bello also uses essential oils to relieve headaches, indigestion and other maladies plaguing herself, her husband and children.

Other lovers of essential oils say they help with body aches, skin conditions and even athlete’s foot.

Among the clients who visit Kelly Holland Azzaro’s massage practice, she says most are seeking relief from sleeplessness and pain. Azzaro is past president and spokeswoman for the National Association of Holistic Aromatherapy, and has been a certified clinical aromatherapy practitioner for 25 years. She uses aromatherapy in her work as a licensed massage therapist at her business in Banner Elk, N.C.

If you want to explore using aromatherapy to get help with physical issues, Azzaro says it’s a good idea to visit a licensed aromatherapy practitioner. And massage therapists should have specific training in the use of essential oils before treating clients with them. It isn’t something that should be approached flippantly, she says.

“I specialize in people with chronic pain, and we often use analgesic oils or anti-spasmatic oils,” Azzaro says. “The first thing we do is go over their health history. It’s very individualized.”

So, with all these people using aromatherapy in their everyday lives, everything’s coming up roses, right? This fragrant trend has some doctors and even aromatherapists a bit concerned. As enthusiasts jump in with both feet, they often haven’t learned about all the potential dangers that come along with essential oil use.

“We at NAHA think education about essential oils is so important, but a lot of people don’t do any research before they start using them. Essential oils come in pretty little bottles, and people think, ‘Well, it smells good, so I’ll use it,’” Azzaro says.

“These are potent essences derived from plant material. They are made up of chemical components. Each has healing properties, but also safety contraindications.”

As people use essential oils to ease certain physical ailments, they risk causing others. For some people, Azzaro says, oils can cause skin rashes, respiratory problems and even sensitization—an issue that arises when someone is over-exposed to a certain scent and gradually develops a sensitivity to it.

Dr. A.J. Eckart is a family medicine doctor of osteopathy practicing in Massachusetts. She decries the use of essential oils, likening them to the snake oil sold in 1800s America.

“The danger in turning to essential oils to alleviate physical issues is the same as the danger in turning to any supplementary complementary alternative medicine,” Eckart says.

“First, there is a delay of care as the patient elects to try an unproven, anecdotal treatment first. This can have devastating effects, say, for the cancer patient eschewing chemotherapy for a more ‘natural’ approach.”

People have a perception, she says, that essential oils are safer than medications. However, the U.S. Food and Drug Administration does not regulate essential oils, so the consumer really has no way of knowing what’s in the bottle.

“A bottle that says ‘100% lavender’ is not always, and mostly rarely, 100% lavender. Worse, herbal remedies often contain contaminants,” she says. “Even if the ingredients written on the bottle are actually in the bottle, they have usually not been adequately tested.”

Using essential oils while taking certain medications can cause adverse reactions, Eckart says. And people often don’t tell their doctor about their essential oil use, which she says can be “quite dangerous.”

Bello says she keeps their family doctors informed about their aromatherapy use, and they have given her the thumbs-up. She says Young Living provides a “seed to seal” purity promise with its products, and she feels confident using them.

“These oils are 100% unadulterated. You get what you pay for, and I firmly stand behind them,” she says.

Inhaled aromas instigate memories, and pleasant ones can put us in a happy frame of mind. Essential oils applied to the skin can have topical effects. But the debate between aromatherapy and traditional medicine is whether essential oils—diffused in air or applied to skin—can help our health.

Eckart says, no, essentials oils don’t actually help alleviate physical ailments.

“There is simply not enough evidence to suggest that aromatherapy is useful for any medical complaint,” she says.

“The healing powers of aromatherapy are entirely based in anecdote and testimonials. Personal experience does not make for a valid scientific claim, unless it has been replicated in a double-blinded, placebo-controlled, large-scale study.”

Some claims, such as those that say certain essential oils boost the immune system, simply don’t even line up with science. Eckart says a healthy person cannot boost his immune system. “That’s just not how the human body works,” she says.

Essential oil distributor doTerra was warned by the FDA in 2014 to stop making medical claims about its products, which are regulated cosmetics and thus cannot carry such assertions. On its website, the company provided comparisons between aromatherapy and prescription medications, and among its laundry list of health claims, recommended using cinnamon oil as a treatment for Ebola virus, clove oil for hepatitis C and oregano oil for norovirus. The company has since removed all claims from its website, literature and sales pitches.

Bello says Young Living is very clear about not making medical claims connected to its products. She always tells clients to talk with their doctors about health issues, and to tell them about their essential oil use.

Despite the dissonance between medicine and aromatherapy, the medical community is starting to open the door just a crack to complementary and alternative approaches, says nurse Cooksley. They’re being won over by thousands of research studies and evidence-based data supporting these types of therapies, she says.

“Among the most remarkable benefits of essential oils are their positive effects on relieving contemporary stress, a major contributing factor in illness today,” she says. “We are also seeing clinical aromatherapy growing in popularity in the prevention and treatment of antibiotic resistant bacterial infections which, unfortunately, are prevalent in American hospitals.”

She says aromatherapy is also being used for psychological first aid, and is entering acute and critical care arenas, in addition to long-term care, hospice and cancer care. Whether it will be a passing trend or a permanent return to an ancient practice is uncertain. Aromatherapy may continue to coexist alongside modern medicine, as the two do have some common relatives. Eckart says modern medications are reliable where essential oils are not, because of the “wild variance in using whole plants.”

“I am not saying that herbal medicine is without benefit. Most of our prescription drugs today have evolved from plants. We continue to find useful medicines in plants,” Eckart says.

“But aromatherapy is a field that relies on hearsay. In science-based medicine, we say, ‘The plural of anecdote is not evidence,’ and that certainly applies here.”