CODEINE occurs naturally in opium, being one of the alka loids (q.v.) of the phenanthrene group and a derivative of mor phine (q.v.), retaining in a modified form the characteristic physi ological action of the latter. It crystallizes with one molecule of water, C18H2103N,H20, in large translucent prisms, melts at C and has a specific rotation [a] It behaves as a mono acidic base, forming salts of which the sulphate and the phosphate are those most frequently used in medicine.