Morphine is the principal alkaloid in opium and the prototype opiate analgesic and narcotic. In 2017, morphine was the 155th most commonly prescribed medication in the United States, with more than four million prescriptions. Morphine is used primarily to treat both acute and chronic severe pain. Its duration of analgesia is about three to seven hours. A large overdose of morphine can cause asphyxia and death by respiratory depression if the person does not receive medical attention immediately. Morphine is naturally produced by several plants (such as the opium poppy) and animals (PMID: 22578954). Morphine was first isolated between 1803 and 1805 by Friedrich Sertürner. Sertürner originally named the substance morphium after the Greek god of dreams, Morpheus, as it has a tendency to cause sleep. The primary source of morphine is isolation from poppy straw of the opium poppy. Morphine is also endogenously produced by humans. In the mid 2000s it was found morphine can be synthesized by white blood cells (PMID 22578954). CYP2D6, a cytochrome P450 isoenzyme, catalyzes the biosynthesis of morphine from codeine and dopamine from tyramine. The morphine biosynthetic pathway in humans occurs as follows: L-tyrosine -> para-tyramine or L-DOPA -> dopamine -> (S)-norlaudanosoline -> (S)-reticuline -> 1,2-dehydroretinulinium -> (R)-reticuline -> salutaridine -> salutaridinol -> thebaine -> neopinone -> codeinone -> codeine -> morphine.  (S)-Norlaudanosoline (also known as tetrahydropapaveroline) which is an important intermediate in the WBC biosynthesis of morphine can also be synthesized from 3,4-dihydroxyphenylacetaldehyde (DOPAL), a metabolite of L-DOPA and dopamine. Morphine has widespread effects in the central nervous system and on smooth muscle (PMID: 4582903). The precise mechanism of the analgesic action of morphine is not fully known. However, specific CNS opiate receptors have been identified and likely play a role in the induction of analgesic effects. Morphine first acts on the mu-opioid receptors. The mechanism of respiratory depression involves a reduction in the responsiveness of the brain stem respiratory centers to increases in carbon dioxide tension and electrical stimulation. It has been shown that morphine binds to and inhibits GABA inhibitory interneurons. These interneurons normally inhibit the descending pain inhibition pathway. So, without the inhibitory signals, pain modulation can proceed downstream. When the dose of morphine is reduced after long-term use, opioid withdrawal symptoms such as drowsiness, vomiting, and constipation may also occur (PMID: 23244430). Morphine is only found in easily detectable quantities in individuals that have used or taken this drug.