Exfoliating the skin
This can be done either mechanically, using an abrasive cloth or a liquid scrub, or chemically. Common chemical exfoliating agents include salicylic acid and glycolic acid , which encourage the peeling of the top layer of skin to prevent a build-up of dead skin cells which combine with skin oil to block pores. It also helps to unblock already clogged pores. Note that the word “peeling” is not meant in the visible sense of shedding , but rather as the destruction of the top layer of skin cells at the microscopic level. Depending on the type of exfoliation used, some visible flaking is possible. Moisturizers and anti-acne topicals containing chemical exfoliating agents are commonly available over-the-counter . Mechanical exfoliation is less commonly used as many benefits derived from the exfoliation are negated by the act of mechanically rubbing and irritating the skin.
Widely available OTC bactericidal products containing Benzoyl peroxide may be used in mild to moderate acne. The gel or cream containing benzoyl peroxide is rubbed, twice daily, into the pores over the affected region and primarily prevents new lesions by killing P.acnes . Unlike antibiotics, Benzoyl peroxide has the advantage of being a strong oxidiser (essentially a mild bleach) and thus does not appear to generate resistance. However, it routinely causes dryness, local irritation and redness. A sensible regimen may include the daily use of low-concentration (2.5%) benzoyl peroxide preparations, combined with suitable non-comedogenic moisturisers to help avoid overdrying the skin. 
Care must be taken when using Benzoyl peroxide, as it can very easily bleach any fabric or hair it comes in contact with.
Other antibacterials that have been used include triclosan , or chlorhexidine gluconate but these are often less effective.
Externally applied antibiotics such as erythromycin , clindamycin , Stiemycin or tetracycline aim to kill the bacteria that are harbored in the blocked follicles. Whilst topical use of antibiotics is equally as effective as oral, this method avoids possible side effects of stomach upset or drug interactions (e.g. it will not affect the oral contraceptive pill), but may prove awkward to apply over larger areas than just the face alone.
Oral antibiotics used to treat acne include erythromycin or one of the tetracycline antibiotics ( tetracycline , the better absorbed oxytetracycline , or one of the once daily doxycycline , minocycline or lymecycline ). Trimethoprim is also sometimes used ( off-label use in UK). However, reducing the P. acnes bacteria will not, in itself, do anything to reduce the oil secretion and abnormal cell behaviour that is the initial cause of the blocked follicles. Additionally the antibiotics are becoming less and less useful as resistant P. acnes are becoming more common. Acne will generally reappear quite soon after the end of treatment-days later in the case of topical applications, and weeks later in the case of oral antibiotics.
In females, acne can be improved with hormonal treatments. The normal combined oestrogen / progestogen contraceptive pills have some effect, but the anti- testosterone Cyproterone in combination with an oestrogen ( Diane 35 ) is particularly effective at reducing androgenic hormone levels. Diane-35 is not available in the USA, but a newer oral contraceptive containing the progestin drospirenone is now available with fewer side effects than Diane 35 / Dianette. Both can be used where blood tests show abnormally high levels of androgens , but are effective even when this is not the case.
If a pimple is large and/or does not seem to be affected by other treatments, a dermatologist may administer an injection of cortisone directly into it, which will usually reduce redness and inflammation almost immediately. This has the effect of flattening the pimple, thereby making it easier to cover up with makeup, and can also aid in the healing process. Side effects are minimal, but may include a temporary whitening of the skin around the injection point. This method also carries a much smaller risk of scarring than surgical removal.
Normalizing the follicle cell lifecycle. A group of medications for this are topical retinoids such as tretinoin (brand name Retin-A), adapalene (brand name Differin) and tazarotene (brand name Tazorac). Like isotretinoin, they are related to vitamin A , but they are administered as topicals and generally have much milder side effects. They can, however, cause significant irritation of the skin. The retinoids appear to influence the cell creation and death lifecycle of cells in the follicle lining. This helps prevent the hyperkeratinization of these cells that can create a blockage. Retinol , a form of vitamin A, has similar but milder effects and is used in many over-the-counter moisturizers and other topical products. Effective topical retinoids have been in use over 30 years but are available only on prescription so are not as widely used as the other topical treatments. Topical retinoids often cause an initial flare up of acne within a month or so, which can be severe.
Reducing the secretion of oils from the glands. This is done by a daily oral intake of vitamin A derivatives like isotretinoin (marketed as Accutane, Sotret, Claravis) over a period of 4-6 months. It is believed that isotretinoin works primarily by reducing the secretion of oils from the glands, however some studies suggest that it affect other acne-related factors as well. Isotretinoin has been shown to be very effective in treating severe acne and can either improve or clear well over 80% of patients. The drug has a much longer effect than anti-bacterial treatments and will often cure acne for good. The treatment requires close medical supervision by a dermatologist because the drug has many known side effects (many of which can be severe). About 25% of patients may relapse after one treatment. In those cases, a second treatment for another 4-6 months may be indicated to obtain desired results. It is often recommended that one lets a few months pass between the two treatments, because the condition can actually improve somewhat in the time after stopping the treatment and waiting a few months also gives the body a chance to recover. Occasionally a third or even a fourth course is used, but the benefits are often less substantial. The most common side effects are dry skin and occasional nosebleeds (secondary to dry nasal mucosa). Oral retinoids also often cause an initial flare up of acne within a month or so, which can be severe. There are reports that the drug has damaged the liver of patients. For this reason, it is recommended that patients have blood samples taken and examined before and during treatment. In some cases, treatment is terminated due to elevated liver enzymes in the blood, which might be related to liver damage. Others claim that the reports of permanent damage to the liver are unsubstantiated, and routine testing is considered unnecessary by some dermatologists. Blood triglycerides also need to be monitored. However, routine testing are part of the official guidelines for the use of the drug in many countries. Some press reports suggest that isotretinoin may cause depression but as of September 2005 there is no agreement in the medical literature as to the risk. The drug also causes birth defects if women become pregnant while taking it or take it while pregnant. For this reason, female patients are required to use two separate forms of birth control or vow abstinence while on the drug. Because of this, the drug is supposed to be given as a last resort after milder treatments have proven insufficient. Restrictive rules (see iPledge Program) for use were put into force in the USA beginning in March 2006 to prevent misuse.  This has occasioned widespread editorial comment. 
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, presumably due to UV damage. More recently, visible light has been successfully employed to treat acne ( Phototherapy ) – in particular intense blue light 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 blue 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.