Butyl Methoxydibenzoylmethane
Also known as Avobenzone, Parsol 1789, Butyl Methoxydibenzoylmethane
“US FDA says: safe within use limit.”
Avobenzone (CAS 70356-09-1) is an organic broad-spectrum UVA filter (340–400 nm) approved in the US at up to 3% (FDA Category I, 21 CFR 352.10) and in the EU under Annex VI at up to 5%. CIR formally deferred its safety evaluation to FDA. The 2025 comprehensive review (PMID 40910486) found NOAEL 450 mg/kg/day in rats, low percutaneous absorption (≤0.59% of applied dose in humans), margins of exposure >100 at 3% use concentration, and no evidence of genotoxicity, carcinogenicity, or endocrine disruption. The primary safety concern is photostability: avobenzone undergoes keto-enol tautomerism and UV-driven C-C bond fission, generating photodegradation products (PMID 36986867). Octocrylene, bemotrizinol (Tinosorb S), and Mexoryl SX are established co-formulant photostabilizers; pairing avobenzone with octocrylene significantly reduces photodegradation (PMID 16289662). A 2020 FDA-authored JAMA study (PMID 31961417) confirmed measurable systemic absorption (lotion: Cmax 7.1 ng/mL), substantially exceeding the FDA 0.5 ng/mL threshold requiring safety studies; however the study concluded these findings should not discourage sunscreen use. Rare photoallergic contact dermatitis cases are documented but not considered a systemic safety concern.
Only FDA Category I broad-spectrum UVA filter (290–400 nm, peak ~355 nm) approved in the US OTC monograph; critical for broad-spectrum SPF labelling compliance
Long history of safe use at 3% (US) and 5% (EU); low acute oral and dermal toxicity; not genotoxic in standard battery
Low percutaneous absorption (≤0.59% of applied dose) confirmed in human studies despite plasma detection — absolute systemic dose remains small at approved use concentrations
Photostabilized formulations (with octocrylene or Tinosorb S) maintain UVA efficacy across extended UV exposure, enabling effective broad-spectrum sun protection
- · Rare photoallergic contact dermatitis: isolated case reports exist; the 2025 comprehensive review characterises this as uncommon and not a systemic safety signal
Photostability: avobenzone is inherently photounstable and degrades under UV exposure via keto-enol tautomerism and C-C bond fission, losing UVA protection over time; photoproducts include p-methoxybenzoic acid and related aromatic fragments — formulation without a photostabilizer results in meaningful SPF/UVA protection loss within a standard application period
Photostabilizer pairing required in practice: typical solutions are co-formulation with octocrylene (triplet-state quencher), bemotrizinol (Tinosorb S), Mexoryl SX, or cyclodextrin encapsulation; without photostabilization the ingredient's UVA efficacy degrades and its photoproducts may pose uncharacterised exposure
Systemic absorption: 2020 FDA JAMA study found plasma Cmax of 7.1 ng/mL (lotion) after a single application, exceeding the FDA 0.5 ng/mL threshold triggering additional safety studies under the Sunscreen Innovation Act; FDA has not concluded that this absorption is unsafe, but additional nonclinical studies are pending
SCCS assessed avobenzone's photoproducts in the context of the EU Annex VI evaluation but no standalone SCCS opinion on the parent compound was identified; the EU permits up to 5% (Annex VI entry 8), which exceeds the FDA 3% ceiling
FDA Federal Register Vol. 62 No. 83 (1997-04-30) — OTC Sunscreen Monograph enforcement policy: avobenzone (butyl methoxydibenzoylmethane)…
“sunscreen drug products containing avobenzone (Parsol 1789) at concentrations of up to 3 percent alone”— Federal Register Vol. 62 No. 83, 1997-04-30, avobenzone enforcement policy section