These drugs are indicated for pain and produce analgesia. Opioid medications have a very large role in palliation in cancer and chronic diseases. They are thought to have similar effects to opiopeptins, natural pain-relievers found in the body.
Because of opioid dependency, patients using opioids need to be weaned, or risk developing a withdrawal syndrome that occurs with abrupt medication cessation. Methadone can be used to wean addicts.
Opioids can be used for cough treatment, as they directly depress the cough center located in the medulla. Codeine may be used to treat severe cough at subanalgesic doses, when not controlled by non-narcotic cough suppressants.
Opioids like loperamide and diphenoxylate can be used to treat persistent diarrhea, as a common side effect of opioids is constipation.
This class of drugs works by mimicking the effects of natural opioids in the body, interacting at the mu (μ), kappa (Κ), and delta (δ) receptors. Each receptor has distinct subtypes, and different drugs may have mixed properties, or partial agonist activity.
Opioids work to open K+ channels and close Ca2+ channels, decreasing synaptic transmission. They inhibit release of acetylcholine, norepinephrine, 5-HT, glutamate and substance P. This can be remembered as decreased pain PANGS: substance P, acetylcholine, NE, glutamate and serotonin).
A possible adverse effect of opioids is respiratory depression, a result of direct inhibition of the brainstem respiratory center. The respiratory center becomes less sensitive to carbon dioxide, leading to decreased hypoxic drive, respiratory rate, tidal exchange, and minute volume.
Psychologic dependence can develop due to the pleasure produced by opioids, especially when self-administered. This may also coincide with physical dependence, as tolerance may build gradually with repeated use. Tolerance varies, and may lead to withdrawal with opioid cessation.
Pinpoint pupils are a side effect, except in meperidine use (often abused by physicians). Opioids act on μ and Κ receptors to stimulate the oculomotor nucleus to constrict pupils.
Opioids may produce constipation, as they decrease gastrointestinal motility and increase tone, specifically of the anal sphincter. There is minimal tolerance to this effect.
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