Retinization

A retinoid speeds up the rate at which skin cells are produced and shed. For the first few weeks the two halves of that process are out of step: the surface sheds faster than barrier lipids can be rebuilt, so skin loses water more easily, flakes at the edges of the treated area, and stings from products it tolerated the week before. That is the adjustment, and it settles as the barrier catches up.

It is worth separating from two things it resembles. Purging is the acceleration of comedones that were already forming, so it shows up as spots in the places you usually break out and fades within about six weeks. An irritant or allergic contact reaction is a different animal altogether: swelling, spreading redness, itching, weeping, or a rash outside the area you applied is not retinization, and it is a reason to stop rather than to persevere.

• Start two or three nights a week and build up, rather than going nightly from the first tube.
• Apply to dry skin, use a pea-sized amount for the whole face, and keep it off the eyelids and the folds beside the nose.
• Buffering — moisturiser before or after — reduces the irritation without meaningfully reducing the result.
• Pause acids and scrubs while adjusting, and wear sunscreen daily.

Irritation that persists past about twelve weeks is not a longer adjustment period; it is a sign the strength or the frequency is wrong for your skin. Dropping a concentration, or moving from tretinoin to a gentler ester, generally buys more improvement over a year than pushing through with a damaged barrier does.