Azelaic Acid

Benzoyl peroxide releases oxygen inside the pore, and the bacteria involved in inflamed acne cannot live in it. It is the fastest of these on red, sore spots, and the one thing that stops acne bacteria becoming resistant to antibiotics.

Salicylic acid dissolves in oil, so it gets down inside the pore rather than sitting on the surface, and loosens the plug of dead skin and sebum blocking the way out. That suits blackheads and whiteheads more than deep, sore spots.

Retinoids change how the pore lining sheds, so plugs stop forming in the first place — which is why they prevent spots rather than treat the ones already there. Given months they also slow pigment reaching the surface and push fibroblasts to build collagen.

Azelaic acid does three things at once: loosens the plug in the pore, reduces the bacteria in it, and calms the pigment cells that leave a dark mark behind. That combination makes it useful on deeper skin tones, where the mark often outlasts the spot.

Niacinamide turns oil production down a little and interrupts pigment being handed to the cells above. The evidence for clearing spots is thin; it earns its place for the marks left behind, and for making stronger actives easier to tolerate.

Clindamycin is an antibiotic that quiets the bacteria in an inflamed pore. Used on its own it stops working within months as they adapt, so it is always paired with benzoyl peroxide or a retinoid.

Spironolactone blocks the hormone signal telling oil glands how much to make. It works on the pattern rather than the individual spot, over three to six months, which is why it suits deep jawline acne that flares with the cycle.

Isotretinoin shrinks the oil glands themselves, and much less oil means less of everything downstream: fewer plugs, fewer bacteria, less inflammation. It is the only acne treatment that often keeps working after the course ends.

The outer layer, and the only one anything in a jar reaches. It renews itself constantly: a cell made at the bottom takes about a month to reach the surface and flake off. Most of what a skincare product does, it does here.

The cells that make pigment. They sit along the base of the epidermis and hand melanin to the cells around them, which is what gives skin its colour. Dark marks, melasma and the patch left behind by a spot are all these cells making more than usual.

A narrow tube running from the surface down into the skin, with an oil gland at the bottom of it. Oil travels up and out. When the tube blocks, what is behind it has nowhere to go — which is where blackheads and spots start.

Makes sebum, the oil that keeps the surface soft and stops water escaping. How much it makes is set by hormones, not by how often you wash — which is why scrubbing does not fix oily skin.

Cutibacterium acnes lives in the pores of everyone with skin. It is not an infection and it is not a hygiene problem. It only causes trouble when a pore blocks and it multiplies in the oil trapped behind it.

The scaffolding in the dermis that keeps skin firm and springy. It is built by fibroblasts and broken down by age, sun and smoking. Lines and looseness are collagen lost faster than it is replaced.

The living layer underneath, holding the blood vessels, the nerves and the collagen. It is where lasting change happens and it is hard to reach: most of what is sold for the skin never gets this far.