Opioids with rigid structural characteristics have generally been found to be effective antitussive agents. The 3-ether derivatives of morphine (i.e. the codeine class) are nearly as effective as morphine for supressing cough, despite substantially reduced analgesic activity. Codeine compounds are frequently used as antitussives due to high efficacy, reduced dependence and their high oral bioavailability. There are several codeine compounds which are currently used or have been used for this purpose. They include codeine, dihydrocodeine, hydrocodone, thebacon, ethylmorphine, benzylmorphine and pholcodine.
Morphine compounds are at least as effective, but are not typically a first choice due to their higher dependence liability and tighter control status. Morphine compounds commonly used for cough include dilaudid, metopon, morphine and heroin. All usually given low dose syrup form. The latter two compounds are obviously not used or available in the US.
Several synthetic compounds are used as well - the narcotics propoxyphene or levopropoxyphene, methadone or isomethadone; as well as the the non-narcotic opioid analogues dextromethorphan and dimemorphan.
Cough supression is not a "true opioid effect", as it is not inherently co-occurring with analgesia and is not antagonized by naloxone. Dextro-isomer opioids retain strong antitussive properties similar to the levo-isomers, despite their virtual absence of mu, delta, or kappa activity. Dextromethorphan is the non-narcotic stereoisomer of the narcotic drug methorphan and retains the antitussive qualities of narcotics. DXM is the gold standard antitussive in the US and elsewhere due to its ability to treat cough without causing morphine-like dependence. It is available without a prescription in most areas.
Opioids relieve diarrhea by reducing gastric motility and producing constipation. Mu and delta receptors located on neurons in the gut play an inhibitory role on gastric motility. Specifically, opioid inhibition of acetylcholine reduces propulsive contractions of the GI tract, while opioid inhibition of adenylate cyclase reduces gastric secretions - Together, these actions produce constipation. The most frequently used antidiarrheal agents are novel compounds sharing structural features of the meperidine and methadone class. These antidiarrheal agents primarily consist of - diphenoxylate, difenoxin and loperamide, the latter of which is for the most part peripherally acting and therefore not a controlled substance in most parts of the world.