Benzoyl peroxide
Benzoyl peroxide is a chemical compound (specifically, an organic peroxide) with structural formula  (C6H5−C(=O)O−)2, often abbreviated as (BzO)2.  In terms of its structure, the molecule can be described as two benzoyl (C6H5−C(=O)−, Bz) groups connected by a peroxide (−O−O−).  It is a white granular  solid with a faint odour of benzaldehyde, poorly soluble in water but soluble in acetone, ethanol, and many other organic solvents. Benzoyl peroxide is an oxidizer, which is principally used as in the production of polymers.As a bleach, it has been used as a medication and a water disinfectant. In specialized contexts, the name may be abbreviated as BPO.
As a medication, benzoyl peroxide is mostly used to treat acne, either alone or in combination with other treatments. Some versions are sold mixed with antibiotics such as clindamycin. It is on the WHO List of Essential Medicines, and, in the US, it is available as an over-the-counter and generic medication. It is also used in dentistry for teeth whitening.
Benzoyl peroxide is also used in the plastics industry and for bleaching flour, hair, and textiles. It is also used to remove stubborn stains from dolls and other plastic toys, although some users have claimed that the product has unwanted long-term effects of discolouring the item, particularly with the My Little Pony brand.

History
Benzoyl peroxide was first prepared and described by Liebig in 1858.  It was the first organic peroxide prepared intentionally.In 1901, J. H. Kastle and his graduate student A. S. Loevenhart observed that the compound made the tincture of guaiacum tincture turn blue, a sign of oxygen being released.  Around  1905, Loevenhart reported on the successful use of BPO to treat various skin conditions, including burns, chronic varicose leg tumors, and tinea sycosis. He also reported animal experiments that showed the relatively low toxicity of the compound.Treatment with benzoyl peroxide was proposed for wounds by Lyon and Reynolds in 1929, and for sycosis vulgaris and acne varioliformis by Peck and Chagrin in 1934. However, preparations were often of questionable quality.  It was officially approved for the treatment of acne in the US in 1960.

Medical uses
Acne treatment
Benzoyl peroxide is effective for treating acne lesions. It does not induce antibiotic resistance. It may be combined with salicylic acid, sulfur, erythromycin or clindamycin (antibiotics), or adapalene (a synthetic retinoid).  Two common combination drugs include benzoyl peroxide/clindamycin and adapalene/benzoyl peroxide, adapalene being a chemically stable retinoid that can be combined with benzoyl peroxide unlike tezarotene and tretinoin. Combination products such as benzoyl peroxide/clindamycin and benzoyl peroxide/salicylic acid appear to be slightly more effective than benzoyl peroxide alone for the treatment of acne lesions. The combination tretinoin/benzoyl peroxide was approved for medical use in the United States in 2021.
Benzoyl peroxide for acne treatment is typically applied to the affected areas in gel, cream, or liquid, in concentrations of 2.5% increasing through 5.0%, and up to 10%. No strong evidence supports the idea that higher concentrations of benzoyl peroxide are more effective than lower concentrations.

Mechanism of action
Classically, benzoyl peroxide is thought to have a three-fold activity in treating acne. It is sebostatic, comedolytic, and inhibits growth of  Cutibacterium acnes, the main bacterium associated with acne.  In general, acne vulgaris is a hormone-mediated inflammation of sebaceous glands and hair follicles. Hormone changes cause an increase in keratin and sebum production, leading to blocked drainage. C. acnes has many lytic enzymes that break down the proteins and lipids in the sebum, leading to an inflammatory response. The free-radical reaction of benzoyl peroxide can break down the keratin, therefore unblocking the drainage of sebum (comedolytic). It can cause nonspecific peroxidation of C. acnes, making it bactericidal, and it was thought to decrease sebum production, but  disagreement exists within the literature on this.Some evidence suggests that benzoyl peroxide has an anti-inflammatory effect as well. In micromolar concentrations it prevents neutrophils from releasing reactive oxygen species, part of the inflammatory response in acne.

Side effects
Application of benzoyl peroxide to the skin may result in redness, burning, and irritation. This side effect is dose-dependent.Because of these possible side effects, it is recommended to start with a low concentration and build up as appropriate, as the skin gradually develops tolerance to the medication. Skin sensitivity typically resolves after a few weeks of continuous use. Irritation can also be reduced by avoiding harsh facial cleansers and wearing sunscreen prior to sun exposure.One in 500 people experience hypersensitivity to BPO and are liable to experience burning, itching, crusting, and possibly swelling.  About one-third of people experience phototoxicity under exposure to ultraviolet (UVB) light.

Dosage
In the U.S., the typical concentration for benzoyl peroxide is 2.5% to 10% for both prescription and over-the-counter drug preparations that are used in treatment for acne.

Other medical uses
Benzoyl peroxide is used in dentistry as a tooth whitening product.

Non-medical uses
Bleaching
Like most peroxides, it is a powerful bleaching agent. It has been used for the bleaching of flour, fats, oils, waxes, and cheeses, as well as a stain remover.

Polymerization
Benzoyl peroxide is also used as a radical initiator to induce chain-growth polymerization reactions, such as for polyester and poly(methyl methacrylate) (PMMA) resins and dental cements and restoratives. It is the most important among the various organic peroxides used for this purpose, a relatively safe alternative to the much more hazardous methyl ethyl ketone peroxide.  It is also used in rubber curing and as a finishing agent for some acetate yarns.

Safety
Explosion hazard
Concentrated benzoyl peroxide is potentially explosive like other organic peroxides, and can cause fires without external ignition. The hazard is acute for the pure material, so the compound is generally used as a solution or a paste. For example, cosmetics contain only a small percentage of benzoyl peroxide and pose no explosion risk.

Toxicity
Benzoyl peroxide breaks down in contact with skin, producing benzoic acid and oxygen, neither of which is very toxic.The carcinogenic potential of benzoyl peroxide has been investigated. A 1981 study published in the journal Science found that although benzoyl peroxide is not a carcinogen, it does promote cell growth when applied to an initiated tumor. The study concluded, "caution should be recommended in the use of this and other free radical-generating compounds".A 1999 IARC review of carcinogenicity studies found no convincing evidence linking BPO acne medication to skin cancers in humans.  However, some animal studies found that the compound could act as a carcinogen and enhance the effect of known carcinogens.

Skin irritation
In a 1977 study using a human maximization test, 76% of subjects acquired a contact sensitization to benzoyl peroxide. Formulations of 5% and 10% were used.The U.S. National Institute for Occupational Safety and Health has developed criteria for a recommended standard for occupational exposure to benzoyl peroxide.

Cloth staining
Contact with fabrics or hair, such as from still-moist acne medication, can cause permanent color dampening almost immediately. Even secondary contact can cause bleaching, for example, contact with a towel that has been used to wash off benzoyl peroxide-containing hygiene products.

Reactivity
The original 1858 synthesis by Liebig reacted benzoyl chloride with barium peroxide, a reaction that probably follows this equation:

2 C6H5C(O)Cl + BaO2 → (C6H5CO)2O2 + BaCl2Benzoyl peroxide is usually prepared by treating hydrogen peroxide with benzoyl chloride under alkaline conditions.

2 C6H5COCl + H2O2 + 2 NaOH → (C6H5CO)2O2 + 2 NaCl + 2 H2OThe oxygen–oxygen bond in peroxides is weak. Thus, benzoyl peroxide readily undergoes homolysis (symmetrical fission), forming free radicals:

(C6H5CO)2O2 → 2 C6H5CO•2The symbol • indicates that the products are radicals; i.e., they contain at least one unpaired electron. Such species are highly reactive. The homolysis is usually induced by heating. The half-life of benzoyl peroxide is one hour at 92 °C. At 131 °C, the half-life is one minute.

References
External links
"Benzoyl peroxide". Drug Information Portal. U.S. National Library of Medicine.
International Chemical Safety Card 0225
NIOSH Pocket Guide to Chemical Hazards. "#0052". National Institute for Occupational Safety and Health (NIOSH).
SIDS Initial Assessment Report from the Organisation for Economic Co-operation and Development (OECD)
Organic Peroxide Producers Safety Division (OPPSD)