Acne develops most often in the teen and young adult years. During this time, both males and females usually produce more testosterone than at any other time in life. This hormone causes oil glands to produce more oil (sebum). The extra oil can clog pores and cause acne. Bacteria can grow in this mixture, and if it leaks into nearby tissues, it causes swelling, redness, and pus (pimples).

Acne usually gets better in the adult years when your body produces less testosterone. Still, some women have premenstrual acne flare-ups well into adulthood.