In Dr. Bowe’s own practice, she said that the fraction of adult female patients with acne jumped from “significant” five years ago to “half of them” three years ago. Today, it’s nearly all of them — to the point that the rare woman without acne is notable.

“That feels like more the exception these days,” Dr. Bowe said.

Not surprisingly, beauty companies have taken notice, introducing blemish products designed for grown-ups. Net-a-Porter’s skin care offerings (of which acne-related products are the “vast majority”) grew 90 percent in the last year; there was triple-digit growth before that, said Newby Hands, the site’s global beauty director.

Doctors don’t yet fully understand what’s causing the recent breakout of … well, breakouts. They think the red angry bumps and pus pimples they usually see on adult women — almost universally in so-called surgical-mask distribution (the lower third of the face) — can be blamed on multiple things. Environmental factors like stress can send oil glands into overdrive, while high glycemic diets have been associated with acne. (So have skim milk and whey protein, though studies on the latter have been done only with men.)

Hormones are another factor. Birth control pills are being prescribed ever younger and more frequently, so when women stop taking them, sometimes 20 years later, it’s the first time the oil glands are subject to the normal cyclical changes of hormones.

“We theorize that the skin is going through a new puberty,” said Joshua Zeichner, the director of cosmetic and clinical research in dermatology at Mount Sinai Hospital. (Why are pimples on the lower third of the face? The current hypothesis is that, as with male pattern baldness, it’s a hormone sensitivity issue.)