Retinoic acid is a physiological compound of human blood. Blood levels range from 1000 to 7000 pg/mL (usually 1500-5000 pg/mL). Results of studies on absorption of topical retinoic acid in laboratory animals, although rather conflicting, demonstrate that it induces plasma concentrations which are well below concentrations caused by non-teratogenic oral doses. In humans, minimal percutaneous absorption of tretinoin was observed after topical applications. Neither single dose nor long-term treatment with topical tretinoin affect the endogenous levels of retinoic acid or its metabolites. Topical application of tretinoin at doses used in acne unlikely induces systemic effects. Although some clinical cases of suspected tretinoin-related embryotoxicity have been described, three prospective cohort studies clearly demonstrated the safety of topical tretinoin as an embryotoxic agent.