Causes of acne

Three factors contribute to the formation of acne:

Overproduction of oil (sebum)
Irregular shedding of dead skin cells resulting in irritation of the hair follicles of your skin
Buildup of bacteria

Acne occurs when the hair follicles become plugged with oil and dead skin cells. Hair follicles are connected to sebaceous glands. These glands secrete an oily substance known as sebum to lubricate your hair and skin. Sebum normally travels up along the hair shafts and then out through the openings of the hair follicles onto the surface of your skin. When your body produces an excess amount of sebum and dead skin cells, the two can build up in the hair follicles and form together as a soft plug, creating an environment where bacteria can thrive.
This plug may cause the follicle wall to bulge and produce a whitehead. Or, the plug may be open to the surface and may darken, causing a blackhead. Pimples are raised red spots with a white center that develop when blocked hair follicles become inflamed or infected. Blockages and inflammation that develop deep inside hair follicles produce lumps beneath the surface of your skin called cysts. Other pores in your skin, which are the openings of the sweat glands onto your skin, aren't normally involved in acne.

Factors that may worsen acne These factors can trigger or aggravate an existing case of acne:

Hormones. Androgens are hormones that increase in boys and girls during puberty and cause the sebaceous glands to enlarge and make more sebum. ...

Certain medications. ...

Diet.

Medications that can caues acne:

Oral corticosteroids, which may also cause steroid acne by increasing yeast proliferation within the hair follicle

Contraceptive agents: medroxyprogesterone injection, implanted  or intrauterine progesterone , and oral contraceptives which reduce circulating sex hormone binding globulin (SHBG), can sometimes aggravate acne in females

TestosteroneAnabolic steroids such as danazol, stanozolol and nandrolone can cause severe acne including acne conglobata and acne fulminans
Athletes and body-builders sometimes abuse anabolic steroids because they result in increased muscle bulk; severe acne can arise in some of these cases.