Cinnamaldehyde

Cinnamaldehyde is the major constituent of cinnamon bark, cassia bark and cassia leaf. The compound is a significant skin sensitizer and irritant, classified as a moderate fragrance allergen.

Skin. A 48-hour patch test at 3 percent caused no irritation; at 8 percent it was severely irritating. Maximation testing at 0.5 percent gave 8 percent sensitization, at 2 percent 44 percent, at 8 percent 32 percent. Cinnamaldehyde elicits concentration-dependent non-immune immediate contact reactions (urticaria).

Across studies in over 25,000 European dermatitis patients, the rate of reactivity to 1–5 percent cinnamaldehyde declined from 1980 to 1996 as concentrations in personal care products fell.

Oral. Acute oral LD50 in rats is 2.2 g/kg; rabbit dermal LD50 exceeds 5 g/kg.

Subchronic. Rat 90-day NOAEL is 215 mg/kg/day by gavage. At 1,075 mg/kg/day for 90 days, forestomach damage and increased mortality occurred.

Reproductive. Gavage 5–250 mg/kg/day in pregnant rats on days 6–15 gave maternal NOAEL of 50 mg/kg/day and developmental NOAEL of 250 mg/kg/day.

Cardiovascular. Antiplatelet aggregation activity, with potential anticoagulant drug interaction.

Mutagenicity. Non-mutagenic in the Ames test with S9; broadly antimutagenic. The compound elicits an adaptive response rather than true genotoxicity.

Recommended dermal maximum is 0.05 percent for sensitization. IFRA limits 0.05 percent in leave-on products.

Cinnamaldehyde distributes 75–85 percent to the GI tract after oral dosing in rats. The major metabolic pathway is degradation to benzoic acid via beta-oxidation and urinary excretion as hippuric acid (82–85 percent).

High doses of 500 mg/kg in rats shift the metabolic profile significantly compared to lower doses, with only 7.6 percent hippuric acid and 73.3 percent benzoic acid.

Cinnamaldehyde is metabolized to cinnamic alcohol and cinnamic acid in human skin. After 10 μL applied to excised female skin under 24-hour occlusion, 9.4 percent was absorbed.

Cinnamaldehyde defines the classical cinnamon aroma. The compound has strong antibacterial activity and serves as a natural preservative.

Over 25 years of clinical data show that reducing concentrations in personal care products effectively reduces population sensitization rates.