The Bottom Line
AHAs (alpha-hydroxy acids like glycolic and lactic acid) are water-soluble and work on the skin's surface — best for sun damage, dullness, fine lines, and dry skin. BHAs (beta-hydroxy acids, primarily salicylic acid) are oil-soluble and penetrate into pores — best for acne, blackheads, and oily skin. Many people benefit from using both, alternating between them.
AHAs: Surface-Level Renewal
Alpha-hydroxy acids dissolve the bonds between dead skin cells on the surface, promoting faster cell turnover. Common AHAs include:
- Glycolic acid: Smallest molecule, deepest penetration. Most effective and most studied AHA. Available at 5-10% for home use, 20-70% for professional peels. Best for sun damage, fine lines, and hyperpigmentation.
- Lactic acid: Larger molecule, gentler penetration. Also acts as a humectant (attracts water). Better for sensitive or dry skin. Good starting AHA for beginners.
- Mandelic acid: Largest AHA molecule, gentlest. Suitable for sensitive skin and darker skin tones (lower risk of post-inflammatory hyperpigmentation).
AHA benefits: Smooths texture, fades dark spots, stimulates collagen (at higher concentrations), improves skin radiance, reduces fine lines.
AHA considerations: Increases sun sensitivity (always use sunscreen). Can cause stinging on first use. Not ideal for active acne with clogged pores.
BHA: Pore-Deep Cleaning
Salicylic acid (the primary BHA) is oil-soluble, meaning it can dissolve into and penetrate the oily interior of pores. This makes it uniquely effective for:
- Acne: Dissolves the mix of oil and dead cells that clogs pores (comedones)
- Blackheads and whiteheads: Clears pores from the inside out
- Oily skin: Reduces excess sebum within pores
- Anti-inflammatory: Salicylic acid is related to aspirin and has inherent anti-inflammatory properties, making it soothing for inflamed acne
Available at 0.5-2% for daily use. Higher concentrations (20-30%) are used in professional peels.
BHA considerations: Can be drying at higher concentrations. Less effective for surface texture and pigmentation than AHAs. Pregnancy safety debated at high concentrations (low-dose topical generally considered acceptable).
How to Choose
| Skin Concern | Best Choice |
|---|---|
| Acne and blackheads | BHA (salicylic acid) |
| Sun damage and dark spots | AHA (glycolic acid) |
| Fine lines and texture | AHA (glycolic acid) |
| Oily, congested pores | BHA (salicylic acid) |
| Dry, dull skin | AHA (lactic acid) |
| Sensitive skin | AHA (mandelic or lactic acid) |
| Acne + aging concerns | Both — alternate nights |
How to Use Safely
- Start low and slow: Use 2-3 times weekly, gradually increasing frequency
- Don't combine with retinoids on the same night: Alternate nights to avoid over-exfoliation
- Always use sunscreen: AHAs especially increase photosensitivity
- Watch for over-exfoliation: Redness, stinging, peeling, or increased sensitivity means you're using too much or too often
- pH matters: AHAs work best at pH 3-4. BHA works best at pH 3-3.5. Check that your product is properly formulated.
Frequently Asked Questions
Can I use AHA and BHA together?
Yes, but carefully. Some products combine both (like The Ordinary's AHA 30% + BHA 2% Peeling Solution — for weekly use only). Alternatively, alternate nights: AHA one evening, BHA the next. Using both daily increases irritation risk.
How long before I see results?
Surface smoothness improves within 1-2 weeks. Acne improvement with BHA takes 4-6 weeks. Significant improvements in pigmentation and fine lines with AHAs take 8-12 weeks of consistent use.
Is "purging" normal when starting acids?
Yes. BHA and AHA can cause an initial "purge" — increased breakouts in the first 2-4 weeks as clogged pores are brought to the surface faster. This is different from a reaction (which causes irritation and redness in new areas). Purging resolves; reactions worsen.
- Tang SC, Yang JH. "Dual effects of alpha-hydroxy acids on the skin." Molecules. 2018;23(4):863.
- Arif T. "Salicylic acid as a peeling agent: a comprehensive review." Clinical, Cosmetic and Investigational Dermatology. 2015;8:455-461.
- Kornhauser A, et al. "Applications of hydroxy acids: classification, mechanisms, and photoactivity." Clinical, Cosmetic and Investigational Dermatology. 2010;3:135-142.