In the United States, codeine is regulated by the Controlled Substances Act. It is a Schedule II controlled substance for pain-relief products containing codeine alone. In combination with aspirin or acetaminophen (paracetamol/tylenol) it is listed as Schedule III. Codeine is also available outside the United States as an over-the-counter drug (Schedule V) in liquid cough-relief formulations. Internationally, codeine is a Schedule II drug under the Single Convention on Narcotic Drugs. In the United Kingdom, codeine is regulated by the Misuse of Drugs Act 1971; it is a Class B Drug, except for concentrations of less than 8mg when combined with paracetamol - or 12.5mg when combined with ibuprofen - which are available in many over the counter preparations. it is a Class B Drug, except for concentrations of less than 8mg when combined with paracetamol - or 12.5mg when combined with ibuprofen - which are available in many over the counter preparations. An opioid analgesic related to morphine but with less potent analgesic properties and mild sedative effects. It also acts centrally to suppress cough. Codeine or methylmorphine is an opiate used for its analgesic, antitussive and antidiarrheal properties. It is marketed as the salts codeine sulfate and codeine phosphate. Codeine hydrochloride is more commonly marketed in contintental Europe and other regions. Codeine is an alkaloid found in opium in concentrations ranging from 0.3 to 3.0 percent. While codeine can be extracted from opium, most codeine is synthesized from morphine through the process of O-methylation. In the United Kingdom, codeine is regulated by the Misuse of Drugs Act 1971; Codeine or methylmorphine is an opiate used for its analgesic, antitussive and antidiarrheal properties. It is marketed as the salts codeine sulfate and codeine phosphate. Codeine hydrochloride is more commonly marketed in contintental Europe and other regions. Codeine is an alkaloid found in opium in concentrations ranging from 0.3 to 3.0 percent. While codeine can be extracted from opium, most codeine is synthesized from morphine through the process of O-methylation. Theoretically, a dose of approximately 200 mg (oral) of codeine must be administered to give equivalent analgesia to 30 mg (oral) of morphine (Rossi, 2004). It is not used, however, in single doses of greater than 60mg (and no more than 240 mg in 24 hours) since there is a ceiling effect. [PubChem]Opiate receptors are coupled with G-protein receptors and function as both positive and negative regulators of synaptic transmission via G-proteins that activate effector proteins. Binding of the opiate stimulates the exchange of GTP for GDP on the G-protein complex. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as substance P, GABA, dopamine, acetylcholine and noradrenaline is inhibited. Opioids also inhibit the release of vasopressin, somatostatin, insulin and glucagon. Codeine's analgesic activity is, most likely, due to its conversion to morphine. Opioids close N-type voltage-operated calcium channels (OP2-receptor agonist) and open calcium-dependent inwardly rectifying potassium channels (OP3 and OP1 receptor agonist). This results in hyperpolarization and reduced neuronal excitability.