The actual cause of acne is not known. Contributing factors include:

• Overproduction of sebum (skin oil)
• Irregular shedding of dead skin cells
• Buildup of bacteria
• Inflammation or infection

There are many factors that influence these contributing factors and outbreaks after onset, including:

• Hormonal changes. These may include changes due to:
  
  
• • Puberty. Involves increased secretion of androgens (male sex hormones) in both males and females, resulting in increased size and activity of sebaceous glands.
    
    
  • Menstruation. Women and girls may experience acne outbreaks two to seven days prior to their periods.
    
    
  • Pregnancy. May improve or worsen acne.
    
    
  • Oral contraceptives. Women starting or stopping birth control pills may experience an improvement or worsening of acne.
    
    
  • Stress. Stress may have an affect on hormones, causing individuals undergoing high levels of stress to experience an outbreak or worsening of acne.
    
    
  • Certain medications. Medications such as corticosteroids, anabolic steroids (such as those used by athletes) and barbiturates may lead to acne outbreaks.
    
    
• Heredity. Severe acne may have a genetic component.
  
  
• Allergies and sensitivities. In some people, food and other allergies or intolerances may cause an outbreak of acne. Also, some individuals are more sensitive to the bacteria that cause acne.
  
  
• Medications. Some people may see increases in acne as a side effect of certain medications.
  
  
• Certain chemicals and other substances. Exposure to halogens (e.g., chlorine, fluorine, iodine, bromine) and tar may lead to acne outbreaks. Direct skin exposure to greasy or oily substances (e.g., from scalp or hair, mineral or cooking oils) and certain cosmetics may cause acne.
  
  
• Friction or pressure on skin. Friction and pressure, such as that caused by telephones, helmets, tight collars and backpacks, may lead to acne outbreaks. Irritation from picking or squeezing acne blemishes, scrubbing the skin too hard or using harsh soaps or chemicals may also cause or worsen acne.
  
  
• Environmental factors. Humidity, pollution, changes in the season and other environmental factors may lead to an outbreak of acne.

Many factors are widely believed to cause or worsen acne when they actually do not. These include:

• Particular foods. Chocolate and greasy foods (e.g., French fries, pizza) are often blamed for acne. In reality, they have no influence on the condition unless an individual has an allergy or intolerance to these items.
  
  
• Poor hygiene. Acne is caused by a build-up of excessive amounts of sebum. Dirt and sweat are not involved. The belief that poor hygiene is related to acne has led many individuals to harshly scrub their skin to get rid of or to prevent acne. This practice can cause irritation and actually make acne worse.