Quick answer: a hair serum is a lightweight leave-in that coats the surface of the hair to smooth frizz, tame flyaways, add shine and make hair easier to comb. Some serums also protect against heat or humidity. It's different from a hair oil, which is mainly plant oils and focuses on softening and nourishing. Use a pea-sized amount or a few drops on the mid-lengths and ends, on damp hair before styling or dry hair to finish.
"Serum" gets used for lots of different products, from silky anti-frizz drops to heat protectants and scalp treatments. This guide is about serums for the lengths of your hair: what they do, how to use one, and whether you need a serum, an oil or both.
What does hair serum do?
- Smooths frizz and flyaways by coating the cuticle so it lies flatter.
- Adds shine, because a smoother surface reflects more light.
- Adds slip so hair detangles and combs more easily, with less snagging and breakage.
- Helps against humidity: many serums form a light barrier that slows frizz on damp days.
- Some protect from heat: a heat-protecting serum is formulated to be used before hot tools. Not every serum does this, so check the label.
- Temporarily seals split ends so they look neater, though only a trim removes them. See split ends: causes and treatment.
What a serum can't do is permanently repair damaged hair or make hair grow. Its effects sit on the surface and wash out.
Hair serum vs hair oil: what's the difference?
| Hair serum | Hair oil | |
|---|---|---|
| Main job | Smooth, shine, frizz control, sometimes heat protection | Soften, nourish and add shine |
| Feel | Usually lighter and silkier | Richer; weight varies by oil |
| Best for | Frizz, flyaways, smooth styles, humidity | Dry, coarse, curly or thirsty lengths |
| When | Before styling or as a finish | As a finish, or as a pre-wash treatment |
The line is blurry: many products, like oil-serums, do a bit of both. If your hair is mainly frizzy, start with a serum. If it's mainly dry, start with an oil. If it's both, you can layer them, using very little of each. More on oils in our hair oil guide.
Which hair serum is right for you?
- For a glossy, polished finish: Liquid Crystals, Best Serum at the HAIR Awards 2022, smooths frizz and flyaways without weighing hair down, with organic sesame and linseed oils.
- For split ends and humidity: Hair Repair Complex helps seal split ends and has a powerful anti-frizz effect that protects the cuticle from humidity, with mosqueta rose and sunflower oils and organic althaea, calendula and mallow extracts. It contains silicones, used because there's currently no naturally derived alternative with the same smoothing performance on the lengths.
- Before heat styling: Replenishing Thermal Care Serum protects up to 230°C with phyto-keratin. See our heat protectant guide.
- For waves, curls and coils: Illuminating Hair Oil-Serum adds shine, softness and elasticity with a blend of rice bran, sweet almond and argan oils plus organic coconut, chia and sacha inchi.
- For fine hair that hates weight: Oil Non Oil, a lightweight anti-frizz fluid with organic aloe, ginkgo biloba and panthenol.
For a detailed comparison focused on frizz, read the best serum for frizzy hair. Shop all hair oils and serums.
How to use hair serum
- Start small: a pea-sized amount or two to three drops for shoulder-length hair. Fine hair needs less; thick or long hair a little more.
- Rub it between your palms to spread it thinly.
- Apply to mid-lengths and ends, then smooth any leftover over the surface. Keep it away from the roots.
- Comb through with a wide-tooth comb for even coverage.
- Style as normal.
Wet or dry hair?
On damp hair, serum helps the blow-dry go smoother and reduces frizz as hair dries (use a heat-protecting formula, or a heat protectant first, if you're using hot tools). On dry hair, a tiny amount smoothed over the surface tames flyaways and adds shine at the end of styling. Many people do both.
Common mistakes
- Using too much, which leaves hair flat or greasy-looking.
- Applying it at the roots.
- Assuming every serum is a heat protectant.
- Layering serum on serum day after day without washing; if hair feels coated, a clarifying wash resets it.
Frequently asked questions
Is hair serum good for your hair?
Yes, for most people. It smooths, adds shine and reduces friction, which helps reduce breakage when you brush and style. It's a surface treatment, not a repair.
Can I use hair serum every day?
Yes, in small amounts. If your hair starts to feel heavy, use less or wash it out more thoroughly.
Does hair serum make hair greasy?
Only if you use too much or put it near the scalp. Start with a few drops on the ends.
Do hair serums contain silicones?
Many do, because silicones are very effective at smoothing. Some, like Liquid Crystals, rely on plant oils instead. Insight is open about where we use silicones and why.
Does hair serum help hair grow?
No. Lengths serums work on the hair you can see. If you're concerned about hair loss, speak to your GP.
Can I use serum and oil together?
Yes. Apply the serum first to smooth, then a drop of oil on the very ends if they're dry. Keep both amounts small.
About the author
Charlotte Read has spent six years in the hair industry with Insight Professional and holds a First Class BSc in Business Management and Marketing. She writes with reference to real salons across social media and the wider Insight community, and draws on the Insight office team, who between them bring over 100 years of combined experience on salon floors.
Where technical expertise is needed, Charlotte consults Lorraine Matic, Head of Insight UK Education, who brings over 38 years of global experience in colour, technique and hair science.