The Collaborative International Dictionary of English v.0.48:

Codeine \Co"deine\ (k[=o]"d[=e]n or k[-o]*d[=e]"[i^]n), n. [Gr.
   kw`deia poppy head: cf. F. cod['e]ine.] (Chem.)
   One of the opium alkaloids; a white crystalline substance,
   C18H21NO3, similar to and regarded as a derivative of
   morphine, but much feebler in its action; -- called also
   codeia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

codeine
    n 1: derivative of opium; used as an antitussive (to relieve
         coughing) and an analgesic (to relieve pain)

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "codeine":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Seconal, Seconal pill,
   Tuinal, Tuinal pill, alcohol, amobarbital sodium, analgesic,
   anodyne, barb, barbiturate, barbiturate pill, black stuff, blue,
   blue angel, blue devil, blue heaven, blue velvet, calmative,
   chloral hydrate, codeine cough syrup, depressant, depressor, dolly,
   downer, goofball, hard stuff, heroin, hop, horse, hypnotic, junk,
   knockout drops, laudanum, liquor, lotus, meperidine, methadone,
   morphia, morphine, narcotic, opiate, opium, pacifier, pain killer,
   paregoric, pen yan, phenobarbital, phenobarbital sodium,
   purple heart, quietener, rainbow, red, scag, secobarbital sodium,
   sedative, shit, sleep-inducer, sleeper, sleeping draught,
   sleeping pill, smack, sodium thiopental, somnifacient, soother,
   soothing syrup, soporific, tar, tranquilizer, turps, white stuff,
   yellow, yellow jacket