opioids

Pain is defined as an unpleasant sensation that can be either acute or chronic and that is a consequence of complex neurochemical processes in the peripheral and central nervous system (CNS).
Alleviation of pain depends on its type. In many cases for example, with headaches or mild to moderate arthritic pain ”nonsteroidal anti-inflammatory agents are effective. Neurogenic pain responds best to anticonvulsants (for example pregabalin), tricyclic antidepressants (for
example, amitriptyline), or serotonin/norepinephrine reuptake inhibitors (for example, duloxetine) rather than NSAIDs or opioids. However, for severe or chronic malignant pain, opioids are usually the drugs of choice.

Opioids are natural or synthetic compounds that produce morphine-like effects. [The term opiate represents drugs, such as morphine and codeine, obtained from the juice of the opium poppy.]

All drugs in this category act by binding to specific opioid receptors in the CNS to produce effects that mimic the action of endogenous peptide neurotransmitters (for example, endorphins, enkephalins, and dynorphins).
Opioid Receptors
Classification of opioid analgesics and antagonists
1- strong agonists: fentanyl, alfentanil, remifentanil, sufentanil, , heroin, meperidine, methadone, morphine, oxycodone,.
2- moderate / low agonists: codeine and propoxyphene.
3- mixed agonist-antagonists and partial agonists: buprenorphine, butorphanol, nalbuphine and pentazocine.
4- Antagonists: nalmefen, naloxone and naltrexone.
5- Other analgesics: tramadol.