The causes of acne are complex reflecting a combination of multiple factors.

Age: Acne is most common in the teenage years and early 20s. It becomes less common with age but can come on for the first time in older individuals.

Genetic factors: Acne and severe acne is more common when other family members have been affected. This reflects an inherited tendency rather than a specific acne gene.

Hormonal factors: The production of sebum is increased by hormonal changes that occur at puberty and this is an important factor in the development of acne. Acne can fluctuate with the menstrual cycle and, where women also have irregular periods and/or excessive hair growth the diagnosis of polycystic ovary syndrome (PCOS) should be considered.

Propionibacterium acnes: This is a common bacteria found on healthy skin, however in individuals prone to acne it can cause infection of hair follicles and worsen the condition.

Drugs: Certain medications including the progesterone only pill 'minipill', steroids and certain novel targeted cancer treatments can trigger the development of acne so it is important to tell your dermatologist about all medications that you are taking. The use of topical steroids, for example for the treatment of eczema on the face can also worsen acne.

Thick or greasy moisturisers or make up can block the opening of the hair follicle triggering the development of acne.

Sustained pressure, for example from a mask, motorcycle helmet or from a weight bench in the gym can compress the openings of hair follicles triggering acne.

Diet: In most cases, diet does not play a significant role in the development of acne however some studies have shown an association between a diet high in milk or high glycaemic index foods.