Acne is caused by a disorder of the sebaceous glands that blocks pores, thus producing an outbreak of the skin lesions. Acne occurs when a change takes place in the inner lining of the hair follicle. What triggers the change is unclear. However, it causes cells that line the hair follicle to be shed too quickly and clump together, ultimately blocking the follicle and preventing the exit of sebum. The resulting mixtures of sebum and clumped cells causes a bacterium (P. acnes), which usually lives on the surface of the skin, to grow inside the blocked follicle. The spots, or pimples, characteristic of acne develop because the skin becomes irritated by inflammation promoting chemicals produced by these bacteria.

The exact cause of acne is still a mystery to scientists. However, changing hormone levels and genetics are known to contribute to the disease:

Rising levels of a class of hormone called androgens can cause the sebaceous glands to enlarge and produce more sebum, thus increasing the risk of acne. Fluctuations in hormone levels are also thought to be responsible for the outbreaks of acne. Women may experience two to seven days before their menstrual period, during pregnancy, when starting ro stopping using the contraceptive pill or when they reach menopause.

There is some reason to suggest that acne may be a hereditary condition. Several studies have found that people with close relatives who suffered from acne are significantly more likely to develop the disease.

Some prescription drugs like androgens, lithium, and barbiturates can cause acne, while stress may worsen the condition.