Ayesha Malik’s long dark hair once fell in such unbelievably perfect Botticelli curls that in 2017 she felt compelled to take to YouTube to prove to online naysayers that she didn’t use a curling iron or wear a wig.

Her hair-care routine included DevaCurl products, which she promoted first as a fan and then, inevitably, as an influencer, flown by the company to New York and Miami from her home in Anchorage to meet fans and post about the brand.

But on Jan. 31 of this year, after many months of bad hair days, itching and hair loss she could no longer hide, Ms. Malik posted a 16-minute talk, titled “Why I Stopped Using DevaCurl,” in which the natural state of her curls appears to be frizzy, as if she’d vigorously brushed them out.

“If you’ve bought DevaCurl products because of me, I am sorry,” she says in the video, which has been viewed more than 2.3 million times on YouTube. “And if you are currently using these products, stop immediately.”

Ms. Malik, 29, is among the most well known of the thousands of women (and a few men) blaming DevaCurl for problems ranging from misshapen and deflated curls to inflamed scalps to hair loss. Stephanie Mero, a hairstylist in Orlando, Fla., who formerly sold (and used) DevaCurl, started a Facebook group called Hair Damage & Hair Loss from DevaCurl — You’re not CRAZY or ALONE. It has nearly 60,000 members, a lot of pictures of thinning hair and bald spots, and a thread for those considering chopping off all their hair. (Ms. Mero has.)

There are also at least 10 class-action lawsuits pending, including four in New York, in which customers say DevaCurl damaged their scalps and made their hair fall out in clumps. (Neither Ms. Malik or Ms. Mero have joined the suits, they said in interviews. Ms. Mero, 29, is mulling her options; Ms. Malik has retained a lawyer and is considering her legal options “because no one’s ever going to get hair advice from me ever again.”)

For a generation of women who grew up straightening their hair, DevaCurl represented a hard-won path to curl acceptance, and customers talked about the brand with religious fervor.

“DevaCurl is the first brand I used that worked well with my hair and sold me on our lord and savior, the CGM,” a Reddit user wrote, referring to the Curly Girl Method of using just conditioner and gel. (CGM was developed by Lorraine Massey, a DevaCurl founder, who left the company in 2013.) For the products not just to fail customers but also to potentially harm them felt akin to betrayal.

The company, which grew out of the success of the SoHo specialty curl salon DevaChan, and whose products were name-dropped on “Broad City,” has established a website called Facts About DevaCurl. There are details of various tests performed and frequently asked questions, including whether DevaCurl is considering a recall. Answer: No.

“Nothing is more important to us than the health of our DevaCurl community,” Jennifer Smith, DevaCurl’s research and development manager, wrote in an emailed statement. She added that based on rigorous testing and consultation with medical professionals, scientists and stylists, “we can conclusively say that our products are safe.”

The company has also offered no-questions-asked refunds, Ms. Smith said. Neither Sephora nor Ulta, both of which carry the products, have immediate plans to remove them.

“We are in close communication with the brand as they continue to update the beauty community on the safety and effectiveness of their products,” Sephora said in an emailed statement. A statement from Ulta also said the company was continuing “to work closely with DevaCurl to understand the issues some users may have experienced.”

Fact-Checking the ‘Facts’

The complaints recall those made against Wen Hair Care, products by the celebrity stylist Chaz Dean, in 2016, when hundreds of people joined a class-action lawsuit claiming that the company’s sulfate-free cleansing conditioners caused hair loss and scalp irritation. (The Food and Drug Administration also investigated, after receiving 127 complaints, the most ever received for a conditioning product.) Wen did not recall its products and denied the allegations but settled the suit for $26.25 million.

It’s still not clear what caused the Deva-Curl problem. On its “facts” website, DevaCurl states: “Our products do not cause hair loss” — that phrase in bold italic — “because they do not penetrate the scalp or affect the hair bulb. Hair loss is often related to excessive scalp irritation, medical conditions and other stressors.”

Asked by The New York Times to fact-check this statement, two doctors — not affiliated with either DevaCurl or the lawsuit — refuted it. “That’s like saying Amazon does not directly influence customers’ holiday spirit because their delivery people do not fully enter buyers’ homes,” said Carlos Wesley, a hair restoration surgeon in Manhattan.

Dr. Wesley, who also reviewed the list of some 200 ingredients posted on the website, said that some of them, including aminomethyl propanol, a product stabilizer, do penetrate the skin’s uppermost layers. And depending on where a hair is in the growing cycle, it can be open or closed to product penetration, making it likely that some cleanser or cream has infiltrated the stem cell region of the follicle, Dr. Wesley said.

Maryanne Senna, a dermatologist and the director of the Hair Academic Innovative Research Unit at Massachusetts General Hospital in Boston, said that products don’t need to reach the hair bulb — the onion-shaped swelling you see if you pull a hair out by the root — to cause hair loss. The superficial inflammation that products may cause can contribute to hair loss and increased breakage.

Inflammation around the follicle can also cause it to harden, affecting hair texture and, in the case of curly hair, curl pattern. What ingredient, specifically, may be causing the irritation? There are multiple possibilities, both doctors and a cosmetic chemist said after reviewing the ingredient list.

Dr. Senna and Perry Romanowski, a cosmetic chemist and founder of TheBeautyBrains.com, a site where scientists examine product ingredients and industry claims, separately suggested that fragrance could be to blame.

“It’s a huge sensitizer,” said Dr. Senna, who also teaches at Harvard Medical School. Dr. Wesley pointed to a solvent called propylene glycol, which causes redness, and said that with such a lengthy list of ingredients, some instances of scalp sensitivity “are not surprising.”

Asked about these scientists’ assessments, DevaCurl responded with a statement from Ms. Smith reiterating that DevaCurl’s products “do not penetrate the scalp or affect the hair bulb.” Hair loss, she added, “can result from many unrelated factors and there is zero evidence that our products are contributing to that process.”

The First Line of Defense

Where does all this leave you when choosing a hair product? Practically needing a degree in cosmetic chemistry, unfortunately.

Both doctors avoided naming brands, saying it was impossible to keep up with the sheer number of products and their ever-changing formulations.

Dr. Wesley recommends washing the scalp at least three times a week to remove sebum, which contains cortisol and DHT, both hormones that contribute to shedding and gradual hair loss. Plain old water — no label reading required there — can work perfectly fine for the task, he said.

When you’re buying products, choose ones that specifically state they’re fragrance-free. Companies are not required to detail ingredients used for fragrance, and many are irritants, Dr. Senna said.

Depending on how concerned you are, you may also want to avoid any of the other ingredients already called out above, as well as a preservative called methylisothiazolinone (MI) and its cousin methylchloroisothiazolinone (MCI), which are sometimes blended. MI had the dubious honor of being named the American Contact Dermatitis Society’s Allergen of the Year in 2013.

As for Ms. Mero and Ms. Malik, both are struggling to replace their former favorite products. “It sucks having to basically date again after the worst breakup of my life,” Ms. Mero said. “That’s what this DevaCurl thing felt like.”