Nobody is completely sure what causes acne. Experts believe the primary cause is a rise in androgen levels - androgen is a type of hormone. Androgen levels rise when a human becomes an adolescent. Rising androgen levels make the oil glands under your skin grow; the enlarged gland produces more oil. Excessive sebum can break down cellular walls in your pores, causing bacteria to grow.
Some studies indicate that a susceptibility to acne could also be genetic. Some medications that contain androgen and lithium may cause acne. Greasy cosmetics may cause acne in some susceptible people. Hormone changes during pregnancy may cause acne either to develop for the first time, or to recur.