According to Wikipedia:

There are many products sold for the treatment of acne, many of them without any scientifically-proven effects. Generally speaking successful treatments give little improvement within the first week or two; and then the acne decreases over approximately 3 months, after which the improvement starts to flatten out. Many treatments that promise big improvements within 2 weeks are likely to be largely disappointing. However short bursts of cortisone can give very quick results, and other treatments can rapidly improve some active spots, but not usually all active spots. Modes of improvement are not necessarily fully understood but in general treatments are believed to work in at least 4 different ways (with many of the best treatments providing multiple simultaneous effects): * normalising shedding into the pore to prevent blockage * killing P. acnes * antinflammatory effects * hormonal manipulation A combination of treatments can greatly reduce the amount and severity of acne in many cases. Those treatments that are most effective tend to have greater potential for side effects and need a greater degree of monitoring, so a step-wise approach is often taken. Many people consult with doctors when deciding which treatments to use, especially when considering using any treatments in combination. There are a number of treatments that have been proven effective:

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. Bar soaps or washes may also be used and vary from 2 to 10% in strength. In addition to its therapeutic effect as a keratolytic (a chemical that dissolves the keratin plugging the pores) benzoyl peroxide also prevents new lesions by killing P.acnes. Benzoyl peroxide often gives clearance rates of 60-70% after 6 weeks, improvements often continue with longer use.[23]Unlike antibiotics, benzoyl peroxide has the advantage of being a strong oxidizer (essentially a mild bleach) and thus does not appear to generate bacterial 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.[24] 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. Prescription-strength benzoyl peroxide preparations do not necessarily differ with regard to the maximum concentration of the active ingredient (10%), but the drug is made available dissolved in a vehicle that more deeply penetrates the pores of the skin.