Thebaine (paramorphine), its name coming from the Greek Θῆβαι, Thēbai, an ancient city in Upper Egypt, is an opiate alkaloid. A minor constituent of opium, thebaine is chemically similar to both morphine and codeine, but has stimulatory rather than depressant effects, causing convulsions similar to strychnine poisoning at higher doses. However the "unnatural" enantiomer (+)-thebaine does show analgesic effects apparently mediated through opioid receptors, unlike the inactive natural enantiomer (-)-thebaine. Thebaine is not used therapeutically but, as the main alkaloid extracted from Papaver bracteatum, it can be converted industrially into a variety of compounds including oxycodone, oxymorphone, nalbuphine, naloxone, naltrexone, buprenorphine and etorphine.
It is controlled under international law, is listed as a Class A drug under the Misuse of Drugs Act 1971 in the United Kingdom and is controlled as a Schedule II of the Controlled Substances Act in the United States of America.
This alkaloid is biosynthetically related to salutaridine, oripavine, morphine and reticuline.