Made for people with chemically straightened or smoothed hair, including keratin treatments, Brazilian blowouts and Japanese straightening, and for anyone who flat irons or blow-dries straight regularly and fights frizz or flyaways.

It is a home-use retail serum. It also works on naturally straight or wavy hair that frizzes in humidity. Those with very fine hair should start with fewer pumps so the ends stay light.

A straightening treatment is an investment, and heat styling and humidity are what wear it down. Hyloren Smoother Hair Serum is built specifically to protect that result: a hyaluronic acid complex replaces the moisture that heat strips away, while a polymer coating guards the strand up to 450°F and cuts the friction that makes the flat iron drag. Mon Platin says this means faster styling, less frizz in damp weather and a shine that looks healthy rather than oily, all in a serum that absorbs quickly enough to use every time you style.

Apply before heat styling on damp or dry hair.

• Pump 2 to 3 times into your hands.
• Spread evenly through the hair, concentrating on the lengths and ends.
• Blow-dry or straighten as usual; leave in.
• For the full Smoother routine, wash with the Hyloren Smoother Shampoo and use the Smoother Hair Mask.

Pros:

• Protects up to 450°F, which covers most flat iron settings used to maintain straightened hair.
• Polymer layer reduces friction, so straightening can take fewer passes.
• Helps smoothing treatments look fresh longer by keeping frizz and flyaways down in humidity.
• SLS-free and positioned by the brand as compatible with keratin and other straightening treatments.

Cons:

• Intended for sleek, straight styles; it will not add volume or hold curls.
• Using too much on fine hair can make the ends look flat, so start with a small amount.
• Full ingredient list not published online.