When you want to give your hair density and body, it makes sense to reach for a “thickening” shampoo. These formulas promise to amp up the appearance of thin strands, so if you’re looking to add some oomph to your look, using one probably feels like a no-brainer. But before you shell out your hard-earned cash, it’s important to understand what they can—and can’t—do.

To find out how hair-thickening shampoos work (and, more importantly, to learn what their limitations are), we chatted with the trifecta of hair-care experts: a stylist, a doctor, and a cosmetic chemist. Keep reading for what they had to say.

How does hair-thickening shampoo work?

“Hair-thickening shampoo contains active ingredients to thicken each hair strand and create a fuller look and feel to the hair’s density,” says Aaron Barry, a hairstylist and makeup artist based in Los Angeles. Unlike volumizing shampoos, which focus on adding lift at the root, thickening formulas do their work all over your head. “Remember, a shampoo is topical and is not going to penetrate the scalp to regenerate dead hair follicles. However, shampoo can stimulate the scalp to promote blood flow to the hair follicles which is essential for healthy hair growth.”

There are two different ways to achieve this thickness. One way is to use ingredients that work inside your strands to plump and thicken them over time. The other way is to use ingredients that leave behind a coating that makes hair look instantly thicker. Some thickening shampoos do both while others do just one.

The ones that work to thicken over time include botanical ingredients like rosemary oil, caffeine, red clover extract, and saw palmetto, explains Michael Wolfeld, MD, a board-certified hair restoration specialist and plastic surgeon. “They’re not going to be as effective as some of the medical treatments that you would use in the office,” says Dr. Wolfeld. “But people have to wash their hair and shower. So, to use something that may help thicken or strengthen their hair is a good idea.” Other common hair-thickening ingredients are Amia Indian Gooseberry, rice amino acids, DHT-blocking pumpkin seed extract, collagen, biotin, and keratin.

The ones that provide instant thickening use ingredients like silicones and guar gums that are engineered to help the product stick to your hair. “The word ‘engineered’ is important, because it’s not just that they drop in an ingredient,” says Joseph Cincotta, PhD, chief chemist at hair-care brand Color Wow. “Combinations of these ingredients have been sought out and tested to stay behind on the hair and not rinse off easily.” As these ingredients coat your hair, they leave behind a film that makes each strand instantly appear thicker.

The instantly thickening shampoos are great, but you want to be careful. “Thickening agents that are left behind on the scalp, that that can then cause inflammation, which can be an irritant,” says Dr. Wolfeld, adding that this can sometimes lead to your hair thinning out even more. The key is to listen to your scalp. “If you find that it’s really irritating the scalp and if you find that you’re having more shedding, then it’s maybe thickening shampoo is something to lay off of.”

How to incorporate hair-thickening shampoos into your routine

When shopping for thickening shampoos, your best bet is to reach for ones that are silicone-free and work over time instead of the ones that promise “instant” results with coating ingredients. And for that instant thickness, use thickening styling products instead of thickening shampoos that coat the hair and scalp.”We have styling products that can [instantly] thicken the hair better than a shampoo,” says Dr. Cincotta. But, if you have an instantly thickening shampoo that you love and don’t want to part, just make sure to alternate with a residue-free shampoo, like Color Wow Color Security Shampoo ($24) or the Zerran Botanum Shampoo ($24) to make sure you’re keeping your scalp and strands clean and ready for nourishment.

And regardless of what kind of thickening products you use, remember that this is more of a band-aid than a solution than a permanent fix. It’s worth repeating that these formulas won’t grow thicker hair. “If you want your hair to look full and thick, you need to get to the root of the cause—literally,” says Marisa Tibbenham, a hairstylist in Los Angeles. “Take care of your scalp—give yourself scalp massages, use oils and products that are beneficial to the scalp. You will see a major difference once you stimulate the scalp.”

The best hair-thickening shampoos to use

Kevin Murphy Plumping Wash — $41.00

“This one provides noticeable volume and actually includes some very great ingredients like ginger root and açaí berry extract that leave your hair looking and feeling incredibly healthy,” says Tibbenham.

Vegamour Gro Revitalizing Shampoo — $48.00

“I love Vegamour shampoo,” says Ashleigh Marie Rancilio, a hairstylist in Los Angeles. “It’s infused with their growth serum and it cleans very well without leaving the hair feeling like there is anything in it.” It’s color-safe and made with 100 percent vegan silk keratin designed to remain attached to your strands even after rinsing.

Alterna Haircare Caviar Anti-Aging Clinical Densifying Shampoo — $38.00

“In my opinion, this is the best for sensitive scalps that need added thickness,” says Barry. “It gently cleanses and detoxifies impurities and buildup and adds volume for thicker-, fuller-looking hair. It contains red clover which helps hair feel thick, strong, and soft. It also includes Amia Indian gooseberry which is commonly used to thicken the look of hair, improve hair health, and reduce scalp irritation.”

 SH-RD Reishi Volumizing Shampoo — $26.00

“This one is great because it is free of sulfates and silicone,” says Tibbenham, which are hard-to-remove ingredients. “It also includes argan oil which promotes a healthy scalp and is said to prevent hair loss.”

BosleyMD BosRevive Non Color-Treated Hair Nourishing Shampoo — $21.00

Barry says this one is great for making hair look thicker while improving scalp health. “It contains pumpkin-seed extract which is a natural DHT-blocking ingredient rich in vitamins and minerals that helps promote healthy hair growth,” he says. “Natural saw palmetto also helps inhibit the action of DHT and may inhibit hair shedding. Rosemary extract’s antioxidant properties help reduce scalp inflammation and increase scalp circulation, creating the ideal foundation for healthy hair growth. Finally, panthenol helps strengthen the hair and improve moisture retention creating thicker, fuller-looking hair.”

If you’re experiencing genetic hair thinning, you can explore these solutions:

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