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.