When an eyelash growth product called Lavish Lash became an Amazon best seller in 2017, a flood of customers promptly wrote to ask: Can I use this on my eyebrows?

Answer: Yes. But then, inevitably, the product’s maker, a Miami company called Hairgenics, released an eyebrow-specific formula. Asked about the key difference in the new product, Mark Transky, the president of the company, said in an interview: “It’s about 15 percent stronger.”

Then he laughed and said, almost ruefully, “Now people are going to use this one on their lashes.”

Such is the desperation for doe eyes and big brows that it seems customers (mostly women) will try almost anything. “There has been a cultural infatuation with brows for some time, along with an increased interest in glowing skin and big lashes,” Dave Kimbell, the president of the Ulta Beauty cosmetic chain, wrote in an email.

It’s a trend that shows little sign of stopping, he said. Besides the many offerings by niche companies like RapidLash, legacy brands like L’Oréal and Maybelline have recently released lash serums.

This is partly because, compared with the hairs on your head, both lashes and brows have super-short growth phases, so it’s tough to catch them then, when they’re treatable.

“You wouldn’t want to yank out all your eyelashes just to get them all to start growing at the same time,” said Angela Christiano, a hair geneticist and a professor of dermatology at Columbia University.

For lashes, there is still only one product proven effective: Latisse, the brand name for a prostaglandin called bimatoprost. The 11-year-old prescription drug, made by Allergan, continues to be popular, in spite of its risks. Besides dry and itchy eyes, side effects include permanent darkening of the skin around the eye and of the iris itself (like from blue to brown).

Users may also see drooping of the upper eyelid and shrinking of the fat pad beneath the eye. (The shrinking may not be a bad thing if you have puffy eyes, but it can make your eyes look sunken if you have deep-set ones.)

There are also over-the-counter potions made with analogues of prostaglandins — often isopropyl cloprostenate, dechloro dihydroxy difluoro ethylcloprostenolamide or methylamido dihydro noralfaprostal on ingredient lists. These are very likely to work, said Maryanne Senna, a dermatologist and the director of the Hair Academic Innovative Research Unit (get it? HAIR) at Massachusetts General Hospital in Boston.

But, she said, “the likelihood of them causing the same problems as Latisse is real.

“That’s my big fear with people buying them, that they won’t be informed about the risk,” Dr. Senna said. (She does prescribe Latisse, though only after a rundown of potential drawbacks.)

A pending federal class-action suit against Rodan + Fields, the maker of the popular Lash Boost (which contains isopropyl cloprostenate), accuses the company of deceptive marketing and said it “failed to disclose the harmful side effects linked to an ingredient.”

A second-class action suit, as well as a personal injury lawsuit, are also pending. Rodan + Fields had no comment.

With any of the prostaglandins, you will need to keep using them or your lashes or brows will go back to whatever they were originally — or, if you have been using the products for years, possibly worse, because of age-related thinning.

Anything without a prostaglandin, no matter how impressive or scientific sounding the ingredients, is unlikely to make lashes grow. Most beauty products in this arena, including ones costing upward of $100, contain “completely inactive bogus ingredients,” said Dr. Senna, who is also an assistant professor of dermatology at Harvard University. (Somewhat tellingly, no doctor interviewed had specific products to recommend.)

What serums like Lavish Lash do, and what may prompt the thousands of five-star reviews, is make lashes look fuller and thicker by coating them with film, said Perry Romanowski, a cosmetic chemist and a founder of https://thebeautybrains.com/, a site where scientists examine product claims.

Mr. Romanowski scoffs at myths companies spin around ingredients like horsetail root and sweet almond protein, saying results actually come from prosaic things like hydroxyethylcellulose, a gelling and thickening agent often found in plain old-fashioned (and much cheaper) mascara.

When it comes to growing brows, there is no product proven specifically effective for them. This doesn’t mean nothing works. Plenty of women successfully use either Latisse or a cheaper option, Rogaine 5 percent, applied carefully with a cotton bud.

Beware of dripping, Dr. Senna said. Otherwise you will have hair growth in areas you don’t want it. (Rogaine works slightly more slowly than Latisse, she said.)

You will get the best results from these products if your brows are naturally thin or if the thinning is, as happens to everyone, aging related. No growth-stimulating product will do anything if you have destroyed the hair follicle by overly aggressive plucking, said Melissa Piliang, a dermatologist at the Cleveland Clinic.

“You could put on all of these products, and the hair around it will respond, but it won’t come back in that little spot,” she said. (Unfortunately, you can’t tell by looking if the follicle has been destroyed. All you can do is try a drug for at least three months.)

Serums and conditioning treatments may make your brow hair look thicker because of similar ingredients to the lash ones, but Mr. Romanowski said that the expensive ones are “pretty much just marketing.” No effective raw material costs that much.Soul Lee, an aesthetician in Manhattan whose clients include Chrissy Teigen, said the current favorite among her customers, nearly half of whom use a serum, is NeuBrow. Ms. Lee herself avoids brow concoctions in a tube. She once tried a RevitaLash solution (it has ones for brows), which, she said, irritated her skin.
“I just fill in every day, and I don’t think about it anymore,” Ms. Lee said. (For lashes, her habitués who are wary of Latisse like GrandeLash serum, which, caveat emptor, contains a prostaglandin analog.)
New treatments may be on the horizon. Dr. Christiano and her colleagues have successfully sprouted new human hairs in the lab, using a multistep recipe that includes skin fibroblasts, collagen, hair follicle dermal cells and 3-D-printed comb-shaped molds that have holes that mimic different hair densities.
Dr. Christiano, who suffers from alopecia areata, an autoimmune disease that causes partial or total hair loss, hopes to be able to implant these proto-hairs back into a patient’s scalp (or possibly, eyebrows).
Another goal of hers is that the “hair farms” the lab creates could help with screening for new hair drugs, something that was previously not possible because there was no way to create human hairs in a dish.
Both Rogaine and Latisse were discovered by accident, happy side effects of drugs originally designed to treat other ailments, minoxidil (Rogaine) for hypertension and bimatoprost (Latisse) for glaucoma.
“There hasn’t been a lot of hair drug discovery because there hasn’t been a way to screen,” she said. “Finally we can break through the bottlenecks.”