Opioids

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An opioid refers to any substance from a group of analgesic agents derived from the ingredient opium. Opioids are a type of depressant, analgesic drug that slow down the messages being sent through the central nervous system between the body to the brain. Although used to treat pain, opioids can entice euphoric feelings and sedative effects which can be addictive which leads this drug group to be commonly abused.

Clinical Pharmacology

Four naturally occurring alkaloids can be isolated from the opium poppy seed (papaver somniferum). These plant-derived amines are morphine, codeine, papaverine and thebaine. These can be used to produce many varieties of semi-synthetic opioids useful in clinical medicine including diamorphine, dihydrocodeine, buprenorphine, nalbuphine, naloxone and oxycodone.

Opioids act on different receptors located on neuronal cell membranes. There are three major types of opioid receptor, m, d and k (mu, delta and kappa). All 3 receptors will produce analgesia when binded with an opioid substance. Activation of k receptors, as with dynorphin, does not produce as much physical dependence as activation of m receptors, as occurs with morphine. The opioid antagonist, naloxone, inhibits all of the opioid receptors which is why its used to treat opioid overdose.

Benefits & Drawbacks of Opioids

Opioids can produce profound analgesia for patients with acute pain. Opioids are so effective at treating acute pain due to their ability to inhibit pain signal transmission at multiple stages in the pain pathway and their ability to enhance the inhibitory pain fibres.

There are many adverse effects associated with opioids. Kappa receptor stimulation is associated with side effects such as hallucinations, dysphoria, anxiety and restlessness. Delta and mu receptor stimulation is often dangerously associated with respiratory depression as the body is unable to regulate carbon dioxide levels in the body effectively. Other drawbacks associated with opioid use in general include sleep apnoea, hypothalamic-pituatory axis supression (hormonal changes with the potential for decreased libido, infertility & fluid retention), urinary retention, nausea & vomiting, physical dependence and addiction, opioid-induced hyperalgesia, dental pathology, constipation, increased mortality and tolerance.

Considerations in Physiotherapy

Opioids are regularly administered for the treatment of pain; many patients that present to physical therapists may have already been prescribed them.