Short answer: Azelaic acid has a half-life of about 12 hours (after use on the skin). People vary, so treat these as estimates.
Azelaic acid is a naturally occurring dicarboxylic acid that is also found in whole grains and animal foods. As a prescription skin medicine it is sold as Azelex 20% cream for acne, and as a 15% gel and Finacea 15% foam for the bumps and pimples of rosacea.
Also known as: Finacea, Azelex.
Azelex cream is 20% (0.2 g per gram). Azelaic acid gel and Finacea foam are 15%; the foam comes in a 50 g can.
The Azelex label approves the 20% cream for mild to moderate inflammatory acne: it is massaged gently into the affected skin twice a day, morning and evening, and the hands are washed afterward. The label says most people with inflammatory acne improve within 4 weeks.
The 15% gel and Finacea 15% foam are approved for the inflammatory bumps and pimples of mild to moderate rosacea. The foam label directions are a thin layer twice a day over the whole face (cheeks, chin, forehead and nose), continued over 12 weeks, with the diagnosis re-checked if there is no improvement; the can is discarded 8 weeks after opening.
The labels warn that skin lightening (hypopigmentation) has been reported, and say people with darker skin should watch for early signs because azelaic acid has not been well studied in them. Allergic reactions, rash and worsening asthma have also been reported. The foam label says so little is absorbed that use in pregnancy is not expected to expose the fetus.
The Azelex label says about 4% of azelaic acid applied to the skin is absorbed into the body. Its half-life is about 12 hours after use on the skin, versus about 45 minutes when taken by mouth, because the slow passage through the skin sets the pace. It leaves mainly unchanged in the urine, and normal blood levels from diet are 20 to 80 ng/mL.
Numbers shown are taken from the source above. Half-life varies from person to person.
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