Opioid

Opioids exert their effects by binding to G-protein-coupled receptors located in the central and peripheral nervous systems. The three main receptor types are mu, delta, and kappa; the mu receptor is primarily responsible for analgesia, euphoria, and respiratory depression. Activation of these receptors inhibits adenylate cyclase, reduces calcium influx, and increases potassium efflux, leading to decreased neuronal excitability and reduced release of neurotransmitters such as substance P and glutamate. This results in pain relief, but also in side effects like constipation, sedation, and respiratory depression. The endogenous ligands for these receptors are enkephalins, endorphins, and dynorphins, which are involved in pain modulation, stress responses, and reward pathways. ¹

Opioids are used for acute pain (e.g., postoperative), chronic cancer pain, and palliative care, but their long-term use for chronic non-cancer pain is controversial due to limited evidence of benefit and high risk of harm. Common adverse effects include nausea, vomiting, constipation, sedation, and pruritus. Respiratory depression is the most dangerous acute effect and can be fatal. Tolerance develops rapidly, requiring dose escalation, and physical dependence leads to withdrawal symptoms upon cessation. Chronic use can cause opioid-induced hyperalgesia, a paradoxical increase in pain sensitivity. ²

The opioid crisis, particularly in North America, has been driven by overprescription of prescription opioids, the emergence of illicit fentanyl, and the proliferation of synthetic analogues. In 2019, an estimated 600,000 deaths worldwide were attributed to opioid overdose, with the majority occurring in the United States and Canada. The crisis has led to public health interventions including prescription drug monitoring programs, naloxone distribution, and medication-assisted treatment with methadone or buprenorphine. However, stigma and policy barriers continue to impede access to effective treatment. ³

Beyond pain relief, opioids have been studied for their effects on the immune system, with evidence that they can modulate immune responses and increase susceptibility to infections. Some opioids, such as tramadol and tapentadol, also inhibit serotonin and norepinephrine reuptake, providing additional mechanisms of analgesia. The poppy plant Papaver somniferum has been cultivated for millennia, and opium was used in ancient civilizations for both medical and recreational purposes. In the 19th century, the isolation of morphine from opium revolutionized medicine, but also led to widespread addiction, prompting the first international drug control treaties in the early 20th century. ⁴

Opioid receptors

G-protein-coupled receptors that mediate the effects of opioids and endogenous peptides.

Tolerance

Reduced response to a drug after repeated use, requiring higher doses to achieve the same effect.

Naloxone

An opioid antagonist used to reverse opioid overdose.