Croton oil (Crotonis oleum) is an oil prepared from the seeds of Croton tiglium, a tree belonging to the order Euphorbiales and family Euphorbiaceae, and native or cultivated in India and the Malay Archipelago. Small doses taken internally cause diarrhea. Externally, the oil can cause irritation and swelling. Croton oil is used in Phenol-croton oil chemical peels for its caustic exfoliating effects it has on the skin. Used in conjunction with phenol solutions, it results in an intense reaction which leads to initial skin sloughing. Since croton oil is very irritating and painful, it is used in laboratory animals to study how pain works, pain-relieving and anti-inflammatory drugs, and immunology.

Because croton tiglium oil is cocarcinogenic, it has been used in tumor research. Croton oil is the source of the chemical compound phorbol. Tumor promotion activity was traced to phorbol esters present in croton oil. Pure phorbol 12-myristate 13-acetate which is found in croton oil is now used widely in laboratory research to induce tumor development.