Codeine is an alkaloid obtained from opium or prepared from morphine by methylation and occurs as white crystals. Codeine effloresces slowly in dry air and is effected by light.

The chemical name of codeine phosphate is 7,8-Didehydro-4,5alpha-epoxy-3-methoxy-17-methylmorphinan-6alpha-ol phosphate (1:1)(salt) hemihydrate and has the empirical formula of C18H21NO3·H3PO4·1/2H20. Its molecular weight is 406.4.

Codeine phosphate is a centrally active analgesic. When administered parenterally, 120 mg of codeine phosphate produces an analgesic response equivalent to that from 10 mg of morphine. Other actions include respiratory depression; depression of the cough center; release of antidiuretic hormone; activation of the vomiting center; pupillary constriction; a decrease in gastric, pancreatic, and biliary secretion; a reduction in intestinal motility; an increase in biliary tract pressure; and an increased amplitude of ureteral contractions.