Acne treatment: Benzoyl Peroxide vs Salicylic Acid

Many OTC topical acne medications contain either benzoyl peroxide or Salicylic Acid.

How do these help with acne?

Why salicylic acid instead of any other acid with similar pKa?

How does benzoyl peroxide work? I never really dealth much with O-O bonds in my chem clases (only peroxyacids), so I’m not really sure what happens here. Do you get radicals when that bond splits?

I guess it’s nice to have two choices in case one has a nasty reaction to one or the other, but is there any other reason why one might be better than the other?

According to, benzoyl peroxide kills the bacteria and salicylic acid decreases skin shedding which clogs follicles.

Since it appears that each is indicated for a different symptom, I wonder why no product contains both. Would it simply be too harsh on the skin to do so, even at less-than-maximum levels?

OTC drugs, including acne products, must be made according to an FDA-approved monograph; there is no combination of benzoyl peroxide and salicylic acid in the monograph. This is most likely because no such combination has ever been tested (because no manufacturer sees any benefit in the product that justifies getting FDA to reopen the monograph). It may also be because, as you say, it would be too harsh.

AFAIK, salicylic acid has not been approved by the FDA as an acne treatment. There is one product called Acne Statin that uses salicylic acid. However, they were fined (story here) for the claims they made with regards to what the product could do.

In my parents’ house there is a tube of “Oil-Free Acne Wash” made by Neutrogena (sp?), which contains salicylic acid. That is the only listed active ingredient. That seems to indicate that it is used to treat acne, but maybe it’s there for another purpose.

I understand that salicylic acid is the main component in wart removers.

Benzoyl peroxide kills bacteria, which helps to reduce the overgrowth that leads to acne. I don’t know its mechanism of action, but that’s the story I’ve heard at least.

Salicylic acid exfoliates the skin - it promotes the shedding of dead skin cells by attacking the matrix that holds the cells together. Remember that craze over hydroxy acids a few years back? This is a beta hydroxy acid, and by removing some of the dead skin cells, it unblocks the pores. Salicylic acid is somewhat oil soluble, which allows it greater penetration into the pores; that’s why it’s more effective than other acids with a similar pKa. It’s also probably got some antiinflammatory properties (it is, after all, closely related to aspirin) which makes in gentler on the skin.

The two together probably would be more effective than either one alone (I use both) but I imagine a single product that combined both would be tough to formulate - apparently salicylic acid is effective only in a sufficiently acidic environment. Plus there’s the issue of irritation - many OTC benzoyl peroxide and salicylic acid products are tremendously irritating to your skin. Which is why they help in the short run, but hurt your skin later. Anything irritating can worsen acne.

By the way, neither one has much effect in the form of a cleanser. They have to be lotions that remain on the skin for a significant period of time.

Ruken, you’re right about the radicals. The O-O bond in benzoyl peroxide is relatively weak and cleaves homolytically (i.e. one electron from the bond goes to each oxygen atom) to form two benzoyloxy radicals. These radicals are good electrophiles and react with the [symbol]p[/symbol]-electrons in double bonds to form a new, more stable radical. These radicals can undergo further reactions or eventually react to become non-radical molecules. Essentially, benzoyl peroxide would kill bacteria by attacking their double bonds and inserting benzylic groups. Chlorine and ozone destroy bacteria in a similar way, by attacking double bonds. Of course, chlorine and ozone aren’t practical to use on the skin, so benzoyl peroxide, a more mild agent, is a better choice for acne medications.

Benzoyl peroxide is used as a free-radical chain polymerization initiator in the synthesis of (among others) poly(vinyl chloride) and poly(styrene) – in both cases the benzoyloxy radical attacks the double bonds in one of the monomers. I don’t recall (and couldn’t look up) exactly what causes the homolytic cleavage of the O-O bond. I suspect it might be UV light.