What is benzoyl peroxide and salicylic acid?

Benzoyl Peroxide
 
Benzoyl Peroxide works as a peeling agent to kill the bacteria that causes acne. It helps regrow new skin after clearing and unclogging the pores and removing dead skin cells. It also dries up existing acne outbreaks. It is often used in over-the-counter acne treatments in concentrations of 2.5% to 10%.

The primary side effect of using Benzoyl Peroxide is skin irritation and dryness. Most people get used to the product after a week or two, but some continue to experience burning, itching, swelling, and peeling. You can ease into it by only using the product every other day until your skin adjusts. Start out with a lower concentration until your skin has adjusted and then increase the concentration. However, if the symptoms continue to occur, you should discontinue use since excessively dry skin can also cause acne.

Benzoyl Peroxide is also a bleaching agent, so be careful when applying it close to eyebrows or the hairline. It is often used in hair dye or teeth whitening. Be careful not to get any on your clothing as it will bleach them. There are rumors that indicate long-term use can cause cancer, but that is so far unsubstantiated.

Salicylic Acid
 
Salicylic acid comes from the bark of a willow tree. It is a beta hydroxyl acid (BHA) that has long been know to have anti-inflammatory properties. Today it is commonly used to treat a number of skin disorders, such as psoriasis, dandruff, calluses, warts, corns, and most importantly, acne. It also works as a peeling agent, clearing away dead skin cells, unclogging pores, and promoting new skin cell growth. By exfoliating dead skin cells, the skin becomes less enflamed and irritated. Blackheads and whiteheads are broken down and the complexion clears up.

Salicylic acid comes in various concentrations from .5% to 2%. The most common side effect is dryness of the skin. This is however, very mild and easily treated with an oil-free moisturizer. If you do experience skin irritation when using products containing salicylic acid, simply reduce the amount of use to every other day or every third day. The problem should clear up quickly. However, there are some products that do not react well when used with salicylic acid. These products include some other topical skin medications, other peeling agents, alcohol-containing preparations, and harsh cleansers and soaps.

(source: http://www.acnetreatment-reviews.com/benzoylperoxide-vs-salicylicacid.html)