Types of Hair Relaxers Lye vs No-Lye vs Thio vs Keratin, How Each Works, and How to Choose

Quick answer: There are four main types of hair relaxers. Lye relaxers (sodium hydroxide) are the strongest and straighten most effectively but carry the highest scalp burn risk. No-lye relaxers (calcium hydroxide or guanidine hydroxide) are gentler on the scalp but can dry hair out over time. Thio relaxers (ammonium thioglycolate) are the mildest chemical option, used for looser curl patterns. Keratin treatments aren’t true relaxers but reduce frizz and loosen curl temporarily. Your choice depends on your curl type, scalp sensitivity, and how straight you want the result.

How All Relaxers Work (The Basic Chemistry)

Last updated: June 12, 2026

Every chemical relaxer works by breaking the disulfide bonds inside the hair shaft. Disulfide bonds are the strong, permanent bonds that give hair its natural curl or coil shape.

When a relaxer breaks these bonds, the hair loses its curl. The hair is then manually smoothed straight while the bonds are broken. When the relaxer is rinsed out and the hair is neutralized (pH brought back to normal), the bonds reform in a straighter configuration.

The differences between relaxer types come down to which chemical does the bond-breaking, how aggressively it works, and what byproducts it leaves behind.

Type 1: Lye Relaxers (Sodium Hydroxide)

Active ingredient: Sodium hydroxide (NaOH), also called lye or caustic soda pH level: 12-14 (extremely alkaline) Straightening power: Maximum

How It Works

Sodium hydroxide is one of the strongest bases used in consumer products. It swells the hair shaft, penetrates the cortex, and breaks disulfide bonds rapidly and thoroughly. The result is the straightest possible finish.

Pros

• Fastest processing time (8-15 minutes for most textures)
• Straightest results, especially on 4B-4C hair
• Doesn’t cause mineral buildup on the hair shaft
• Easier to rinse out completely

Cons

• Highest risk of chemical burns on the scalp
• Requires petroleum jelly (base) on the scalp before application
• Most damaging to hair structure per application
• Can cause permanent hair loss if left on too long or applied to previously relaxed hair

Best For

Type 4 hair (4A-4C) that wants maximum straightness and has a healthy scalp. Professional application strongly recommended.

Type 2: No-Lye Relaxers (Calcium Hydroxide / Guanidine Hydroxide)

Active ingredient: Calcium hydroxide mixed with guanidine carbonate to form guanidine hydroxide pH level: 9-11 (still alkaline, but lower than lye) Straightening power: Strong

How It Works

No-lye relaxers use a two-part mixing system. You combine a calcium hydroxide cream with a guanidine carbonate activator. The resulting guanidine hydroxide breaks disulfide bonds more slowly than sodium hydroxide, which gives a wider margin for error during application.

Pros

• Lower scalp irritation and burn risk
• Available in home kits (easier for self-application)
• Gentler processing, less likely to cause immediate damage
• Good option for sensitive scalps

Cons

• Deposits calcium on the hair shaft over time, causing dryness and dullness
• Requires regular clarifying or chelating treatments to remove calcium buildup
• Doesn’t straighten as completely as lye (some texture may remain on 4C)
• Longer processing time (15-25 minutes)

Best For

People with sensitive scalps, first-time relaxer users, and those applying at home. Works well on Type 3C-4B hair.

No Lye Hair Relaxer

Type 3: Thio Relaxers (Ammonium Thioglycolate)

Active ingredient: Ammonium thioglycolate (the same chemical used in perms) pH level: 8.5-9.5 (mildly alkaline) Straightening power: Moderate

How It Works

Thio relaxers use a reducing agent instead of a strong base. Ammonium thioglycolate breaks disulfide bonds by donating hydrogen atoms rather than by extreme alkalinity. The result is a gentler, more gradual loosening of curl.

This is the same chemistry used in permanent waves, just applied differently. In a perm, the bonds are reformed around a rod (creating curls). In a thio relaxer, the bonds are reformed while the hair is held straight.

Pros

• Gentlest chemical option of the three true relaxers
• Lowest damage per application
• No calcium buildup (unlike no-lye)
• Good for loosening curl without going completely straight

Cons

• Weakest straightening power; won’t fully straighten 4B-4C
• Distinctive sulfur smell during and after processing
• Results aren’t as permanent feeling (curls may return more quickly)
• Less commonly available in consumer kits

Best For

Type 2C-3C hair that wants to loosen curls rather than eliminate them entirely. Also used for “texturizing” (intentionally partial relaxing to make coils more manageable without going bone-straight).

Type 4: Keratin Treatments (Not a True Relaxer)

Active ingredient: Varies. May contain formaldehyde, glyoxylic acid, or glycolic acid pH level: 2-4 (acidic) Straightening power: Mild to moderate

How It Works

Keratin treatments coat the hair shaft with a layer of protein (keratin) and then seal it with high heat (flat iron at 400-450°F). This smooths the cuticle and loosens curl. Unlike true relaxers, keratin treatments don’t permanently break disulfide bonds. The results wash out over 2-5 months.

Pros

• Temporary (washes out, so not a permanent commitment)
• Dramatically reduces frizz
• Adds shine and smoothness
• Less structural damage than chemical relaxers

Cons

• Many formulas contain formaldehyde or formaldehyde-releasing agents, which are respiratory irritants and classified as carcinogenic by IARC
• Requires professional application ($150-400 per session)
• Must avoid water for 48-72 hours after treatment
• Needs to be redone every 2-5 months ($600-1,600 per year)
• Won’t fully straighten tight 4C coils

Best For

People who want reduced frizz and looser texture without permanent chemical change. Popular with Type 2-3 hair. Not strong enough for Type 4 hair that wants straight results.

Side-by-Side Comparison

How to Choose Your Relaxer Type

Choose lye if: You have 4B-4C hair, want maximum straightness, have a healthy scalp, and plan to have it applied by a professional.

Choose no-lye if: You have a sensitive scalp, want to apply at home, or are relaxing for the first time. Be prepared to use a chelating shampoo monthly to prevent calcium buildup.

Choose thio if: You have 2C-3C hair and want to loosen (not eliminate) your curl pattern. Or if you want a “texturized” look that’s more manageable but not bone-straight.

Choose keratin if: You want reduced frizz and looser texture without permanent commitment. You’re willing to spend $600+ per year. You want something that washes out if you don’t like it.

Hair Relaxer Kit

Safety Rules for All Relaxer Types

1. Never overlap. Apply relaxer only to new growth, never to previously relaxed hair. Overlapping causes severe damage and breakage.
2. Base your scalp. Apply petroleum jelly to the scalp, hairline, and ears before application (especially with lye relaxers).
3. Watch the clock. Set a timer. Leaving a relaxer on past the recommended time causes irreversible damage.
4. Neutralize fully. The neutralizing shampoo step brings pH back to normal and stops the chemical reaction. Skipping or rushing this step means the relaxer continues breaking bonds.
5. Wait 6-8 weeks between touchups. You need enough new growth (at least 0.5-1 inch) to apply to without touching previously relaxed hair.

Frequently Asked Questions

Q: What are the different types of hair relaxers? A: Four types. Lye (sodium hydroxide, strongest), no-lye (guanidine hydroxide, medium strength), thio (ammonium thioglycolate, mildest chemical), and keratin treatments (temporary smoothing, not a true relaxer).

Q: Is lye or no-lye relaxer better? A: Neither is universally better. Lye straightens more effectively but burns the scalp more easily. No-lye is gentler on the scalp but causes calcium buildup that dries hair over time. Choose based on your scalp sensitivity and straightening goals.

Q: Can I switch from lye to no-lye? A: Yes, but not in the same session. Wait until you have enough new growth and apply the new relaxer type only to new growth. Never mix two relaxer types on the same hair, as the chemical interactions can cause severe breakage.

Q: Which relaxer is least damaging? A: Thio relaxers cause the least structural damage, but they also straighten the least. If you need strong straightening, no-lye is less damaging than lye for comparable results. Keratin treatments are the least damaging overall but aren’t permanent.

All chemical relaxers permanently alter hair structure. Understanding which type matches your texture, scalp, and lifestyle prevents damage and disappointment. When in doubt, consult a licensed stylist who specializes in chemical services for your hair type.

For specific product recommendations, see our relaxer for Black hair guide. For pre-relaxing prep, see our clarifying before relaxing guide.