Acne occurs when oily secretions, also known as sebum, and dead skin cells, accumulate in the hair follicles and pores of the skin. Bacteria called Propionibacterium acnes (P. acnes) can infect the clogged pore, feed on the sebum and cause inflammation. If a hard, painful cyst forms from the debris and bacteria that feed on it, this acne is called cystic acne.

Low intensity blue light can kill P. acnes, the bacteria that infects clogged pores to create cystic acne. Molecules produced by the bacteria are changed into free radicals by the blue light and go on to destroy the bacteria that created them. A 2005 study published in the "Journal of Drugs in Dermatology" found that blue light therapy in combination with aminolevulinic acid (ALA) was highly effective in treating cystic acne in an Asian case study. The United States Food and Drug Administration has approved a specific high-intensity narrow band blue light therapy for the treatment of acne.

Treatment using blue light therapy for cystic acne is usually done once or twice a week for up to eight weeks. A single session takes between eight and 20 minutes. Treatment can be done in a dermatologist's office or at home, using home kits. Topical treatments, such as ALA can be used along with blue light therapy to increase the effectiveness and destroy more of the harmful bacteria. Some systems also use a red light in conjunction with blue light therapy to help ease inflammation and swelling.

The cost can range between $40 and $200 per session to over $1,000 for a full series of in-office treatments that also include ALA application before the light therapy treatment. A handheld home unit can cost from $300 to $400.

Blue light therapy for cystic acne is convenient and quick, since it can be scheduled in a doctor's office or done on a set schedule at home. It also tends to be gentle on the skin, which can be especially important for people with sensitive skin who react poorly to harsh topical acne treatments.

Because the bacteria that cause cystic acne multiply quickly, multiple treatments are usually needed. The cost can be another disadvantage, since creams, antibiotics and other methods of treating acne tend to be less expensive than blue light therapy.

Potential side effects are mild, but include temporary redness and dryness of the treated skin. However, because blue light therapy to treat cystic acne is a relatively new process, there may be potential side effects that are still undiscovered. Also, blue light therapy treatment can only help with acne that is cased by bacterial infection. Other types of acne will not be affected by using this type of treatment.