• Hormonal activity, such as menstrual cycles
• Stress, through increased output of hormones from the adrenal (stress) glands
• Hyperactive sebaceous glands, secondary to the three hormone sources above
• Accumulation of dead skin cells
• Bacteria in the pores, to which the body becomes 'allergic'
• Skin irritation or scratching of any sort will activate inflammation
• Anabolic steroids - the body-building steroids
• Any medication containing halogens (iodides, chlorides, bromides), lithium, barbiturates, or androgens
• Exposure to high levels of chlorine compounds, particularly chlorinated dioxins, can cause severe, long-lasting acne, known as Chloracne

 Traditionally, attention has focused mostly on hormone-driven over-production of sebum as the main contributing factor of acne. More recently, more attention has been given to narrowing of the follicle channel as a second main contributing factor. Abnormal shedding of the cells lining the follicle, abnormal cell binding ("hyperkeratinization") within the follicle, and water retention in the skin (swelling the skin and so pressing the follicles shut) have all been put forward as mechanisms involved.

Several hormones have been linked to acne: the male hormones testosterone, dihydrotestosterone (DHT) and dehydroepiandrosterone sulfate (DHEAS), as well as insulin-like growth factor 1 (IGF-I). In addition, acne-prone skin has been shown to be insulin resistant.