AHA & BHA

Both loosen the glue holding dead skin cells on the surface. The difference is where they can reach.

• AHAs such as glycolic, lactic and mandelic acid are water loving. They work on the surface, so they suit dullness, rough texture and uneven tone
• BHAs, which in practice means salicylic acid, are oil loving. They get down inside the pore, so they suit blackheads, congestion and blemishes
• PHAs are the gentler third option. Larger molecules, slower acting, much less likely to sting

Dry or dull skin usually does better on an AHA. Oily or congested skin usually does better on a BHA.

Start with a wash-off product. It gives you the ingredient with far less risk while you find out how your skin reacts.

• Dullness and rough texture: SkinCeuticals Glycolic Renewal Cleanser or Reform Skincare Glycolic Acid Foaming Cleanser
• Congestion and blackheads: Obagi CLENZIderm Daily Foaming Cleanser or Reform Skincare Salicylic Acid Foaming Cleanser
• Easily irritated skin: Medik8 Press & Glow, which is the gentler PHA option

Once that is comfortable, a leave-on toner is the natural next step.

Two or three times a week is right for most people. Every day is too much for almost everyone, whatever the bottle says.

Start once or twice a week and add a night only when your skin is completely comfortable at the pace you are on. Going up too fast is the single most common mistake with acids.

A wash-off acid can go more often than a leave-on, because it is on your skin for seconds rather than hours.

Not all in the same routine. Space them out and you get the benefit of each without wrecking your barrier.

• Vitamin C in the morning, acids at night
• Retinol and acids on alternate nights, never layered on the same night
• Acids and a strong mask on the same day is too much. Pick one

If you are already using a prescription retinoid, talk to whoever prescribed it before adding an acid.

A wash-off acid is the safer, slower route. A leave-on does more, and asks more of your skin.

Cleansers like Obagi CLENZIderm Daily Foaming Cleanser rinse away in under a minute. Leave-on toners and pads like Anua BHA 2% Gentle Exfoliating Toner, Image Skincare Clear Cell Salicylic Clarifying Pads and Medik8 Press & Clear stay on and keep working.

Most people do best with one of each rather than two leave-ons. A weekly treatment like Image Skincare Ageless Total Resurfacing Masque covers the deeper end without adding another daily step.

A brief tingle on application is normal. Stinging that lasts, redness, tightness or flaking means you have overdone it.

Stop the acid completely and go back to a plain cleanser, a moisturiser and SPF for two weeks. Do not try to fix a stripped barrier with more product.

When you start again, halve the frequency you were on. Acids also make skin more vulnerable to sun, so daily SPF is not optional while you are using them.