There are different types of acne. The most common acne is the type that develops during the teen years. Puberty causes hormone levels to rise, especially testosterone. These changing hormones cause skin glands to start making more oil (sebum). Oil releases from the pores to protect the skin and keep it moist. Acne begins when oil mixes with dead cells  and clogs the skin's pores. Bacteria can grow in this mixture. And if this mixture leaks into nearby tissues, it causes swelling, redness, and pus. A common name for these raised bumps is pimples.

Certain medicines, such as corticosteroids or lithium, can cause acne to develop. Talk to your doctor about any medicines you are taking.