Phototherapy
'Blue' and red light
It has long been known that short term improvement can be achieved with sunlight. However, studies have shown that sunlight worsens acne long-term[citation needed]. More recently, visible light has been successfully employed to treat acne (phototherapy) - in particular intense violet light (405-420nm) generated by purpose-built fluorescent lighting, dichroic bulbs, LEDs or lasers. Used twice weekly, this has been shown to reduce the number of acne lesions by about 64% and is even more effective when applied daily.

The mechanism appears to be that a porphyrin (Coproporphyrin III) produced within P. acnes generates free radicals when irradiated by 420nm and shorter wavelengths of light.Particularly when applied over several days, these free radicals ultimately kill the bacteria.Since porphyrins are not otherwise present in skin, and no UV light is employed, it appears to be safe, and has been licensed by the U.S. FDA.

The treatment apparently works even better if used with red visible light (660 nanometer) resulting in a 76% reduction of lesions after 3 months of daily treatment for 80% of the patients; and overall clearance was similar or better than benzoyl peroxide. Unlike most of the other treatments few if any negative side effects are typically experienced, and the development of bacterial resistance to the treatment seems very unlikely.

After treatment, clearance can be longer lived than is typical with topical or oral antibiotic treatments; several months is not uncommon. The equipment or treatment, however, is relatively new and reasonably expensive to buy initially, although the total cost of ownership can be similar to many other treatment methods (such as the total cost of benzoyl peroxide, moisturiser, washes) over a couple of years of use