Cardi B recently set social media ablaze when she took to Instagram to share a potentially unexpected secret to healthy hair: onion water.

The rapper says that the DIY, at-home treatment has left her hair shinier after just two washes — and the supposed before-and-after pictures she posted to Instagram are undeniably convincing.

She may be the first female rapper to have three diamond single certifications from the Recording Industry Association of America, per Paper Magazine, but she's certainly not the first to use the at-home hair treatment. Countless followers who flocked to Cardi B's comments section praised the performer for utilizing the natural remedy, as it's deeply embedded in ayurvedic practices and Dominican culture (though it’s unclear where the practice originated). And William Gaunitz, a certified trichologist, or hair and scalp expert, and founder of Advanced Trichology in Phoenix, Arizona, says that his hair loss patients have been discussing this natural treatment for years.

Gaunitz says claims about onion water's benefits for the hair "hold validity to a degree." But he doesn't unconditionally support the practice because "it's subject to a random recipe" with a "variety of variables" that may or may not work.

While Cardi B's hack involved boiling onions and using the water, Gaunitz notes that he’s encountered patients who’ve blended onions to make a juice-like substance that they use as a scalp treatment before rinsing it off in the shower.

Still others chop the vegetable and add it to their shampoo. That's the case for PureWow writer Angie Martinez-Tejada, who learned about the supposed trick from her Dominican mother. She used the onion shampoo first and followed up with a second shampoo, which she says masks any lingering onion scent.

But what does the science say about this hair-raising hack? Ahead, we break down everything you need to know before you douse your hair in onion water.

What Are the Health Benefits of Onion Water for Hair?

Onions’ health properties mean the vegetable may offer a bevy of benefits for the hair. "Rich in antioxidants and flavonoids, onions offer antimicrobial, antifungal, and anti-inflammatory benefits," says Kerry Yates, a trichologist and founder of Colour Collective based in Dallas. These properties make it a potential solution to alleviate fungal infections that can cause dandruff, she adds. While there’s a lack of high-quality research on how these properties translate to your scalp, one review published in 2020 in the Journal of Pharmacy and Bioallied Sciences noted the antifungal and antimicrobial potential of onion extract and onion essential oils.

And while research on the real-world applications of onions for hair is limited, one past study suggested that onion water could help promote hair re-growth among study participants living with alopecia areata, which is a condition that causes hair loss.

The study involved 38 women and men who applied onion juice or tap water to their scalp twice daily for two months. Of the 23 participants who used onion juice, 20 people (or 87 percent) noticed regrowth by six weeks into the trial, while only two people from the tap water group noticed the same. But the small sample size of this study means that further research is needed to fully understand this potential effect.

One possible reason onion juice may be a boon to your hair? "The sulfur content of onion water is very high," says Gaunitz. You’ll also find high levels of sulfur in other foods, including meat, eggs, cruciferous veggies, and other alliaceous veggies like garlic, per past research. This nutrient leaves you teary after chopping onions, noted The New York Times. But it can have other effects, Gaunitz continues: "Since sulfur is a common OTC treatment for inflammatory skin conditions when applied to the scalp, it works for many people who have inflammatory hair loss issues.”

Take note, though: Onion water won't work for all types of hair loss, such as hair loss caused by nutrient deficiencies and androgenetic alopecia, says Gaunitz. There’s no research to suggest that onion juice could suppress dihydrotestosterone levels to prevent androgenetic alopecia. Likewise, it hasn’t been shown to boost protein or vitamin D levels at the base of the hair follicle, which could assist in nutritional hair loss.

In either of those cases, it's best to consult with a certified dermatologist or trichologist.

What Are the Potential Side Effects of Onion Water for Hair?

If you are allergic to onions, avoid using them topically entirely, including in onion water, warns Yates.

And even if you’re not allergic, be sure to thoroughly wash the onion water out of your scalp to prevent irritation. (It’s unlikely, but not impossible, that it could irritate your skin, according to the Environmental Working Group.)

While smelling like onions isn't a health-related side-effect, it's probably not the fragrance you’re after. The good news: Teamed with shampoo, "the onion smell should come right out" after rinsing, says Gaunitz.

How to Use Onion Water for Healthy Hair

If you're interested in giving the onion water hair remedy a try, Gaunitz advises against cooking or boiling them. “The [theoretical] medicinal properties come from the raw onion itself," he explains.

Instead, here’s one potential method, per the New Jersey Hair Restoration Center:

Peel and chop three to four onions into small pieces.
Extract the juice by squeezing or blending them.
Apply the juice to your scalp using a cotton pad. Optionally, adds Yates, you can add a little lemon juice if you want to offset the smell of the onion.
Massage it into the scalp, let it sit for an hour, and then rinse. Follow with a second cleanse of your usual shampoo.

If you're wary about concocting a DIY onion water solution, Yates suggests opting for a topical haircare treatment like shampoo, conditioner, or oil containing onion juice.

Hair-Healthy Alternatives to Onion Water

Not all experts advocate for the onion water trend. "While onions can be a powerful antioxidant, I wouldn't invest too much stock, medically speaking, in the latest social media trend, or one that celebrities swear by," says William Yates, MD, a Chicago-based hair loss surgeon. “The truth is that much of your hair loss (or lack thereof) is genetically written in stone. Aside from disease, which we saw happen very widespread with COVID, and hormonal imbalances, your hair density is mostly predetermined based on the inherited traits from your parents.”

Instead, he advises that you focus on "getting enough nutrients by eliminating processed foods and opting for a diet rich in protein, vitamins, and minerals." The Cleveland Clinic offers similar advice to prevent hair loss, noting that protein-rich foods and a Mediterranean diet may be especially beneficial.

As for your hair-care routine: "Remove any harsh chemicals such as phthalates, sulfates, and alcohol," Dr. Yates advises.

The Takeaway

Using onion water to improve scalp health and your hair’s appearance is not a new practice — it’s rooted in Ayurvedic practices and Dominican culture. Overall, while some people claim it can reduce dandruff, reduce hair loss, and boost shine, the research evidence to support widespread use of onion treatments for scalp and hair issues is lacking. Although the theoretical risk and cost are low, consult a dermatologist or trichologist before trying the DIY treatment, as not all hair concerns can benefit from this practice.