What are opioids?

We’ve all heard of opioids. Many of us have prescribed them and most of us have met patients using them. But what are they and why does this matter to our patients? In this article, Dr Harriet Scott, Specialty Registrar in Anaesthesia and Pain Medicine at Barts Health, helps to answer these questions.

Opioids are chemicals which are agonists at opioid receptors.

This means that when an opioid binds to an opioid receptor there is an action enacted by that receptor. Opioid receptors are found throughout the body but are mostly concentrated in the central nervous system. The action of opioids at opioid receptors causes a reduction in pain signalling and perception, and this makes opioids particularly useful as analgesics (drugs providing pain relief).

A note on terminology. Opioid is an umbrella term used for all synthetic and natural substances that have agonist activity at opioid receptors. The term opiate refers to all naturally occurring substances with morphine-like properties derived from the natural alkaloids of the opium poppy, Papaver somniferum.

History of opioids

There is evidence of opioids being used by ancient civilisations including the Sumerians in Mesopotamia (encompassing modern-day Iraq and Kuwait) from around 3400 BCE, where the opium poppy, Papaver somniferum, was known as ‘the joy plant’. There is also evidence of opium cultivation being practised by the ancient Greeks, Persians and Egyptians, who used opium in remedies to aid sleep, relieve pain and even soothe crying children.

The Opioid Crisis of the 21st century is not the first such crisis history has seen. A network of trade routes, known as the Silk Road, connecting East Asia with Europe and Southwest Asia, first provided a route of opium supply from the Caucasus into China in the 6th century CE. Opium use was limited until the 1800s, when rates of addiction rose sharply in China due to opium smuggling by the British through the East India Trading Company, causing the Qing Dynasty to outlaw opium importation and cultivation. This led to the two Opium Wars, as China battled to keep opium out of its borders and Britain fought to keep lucrative trafficking routes open. The first war took place in 1839-1842, leading to China ceding Hong Kong to the British, whilst the second took place in 1856-1860. The British victories led to unrestricted trading of opium around the world and a rise in the documented prevalence of addiction at the time.

_{Morpheus Sleeping (1782) – Ivan Prokof’yevich Prokof’yev – State Russian Museum, St. Petersburg (via Web Gallery of Art).}

By the beginning of the 19th century, medical opium had been available for centuries in Western Europe as a number of different preparations, including laudanum, a tincture of opium which was used in the management of pain, insomnia and diarrhoea. The chemical isolation of morphine itself took place in 1804, when it was named after Morpheus, a god related to dreams and sleep in ancient Greek mythology. The subsequent introduction of the hypodermic needle and syringe in the early 1850s popularised injected morphine as an analgesic.

Morphine use, both orally and injected, was prevalent during the American Civil War in 1861-1865, causing widespread addiction among servicemen.

Diamorphine (also known as diacetyl morphine or by its original trademark name, Heroin) was manufactured by Bayer in 1898 and marketed as a treatment for cough and morphine addiction. As diamorphine’s own addictive properties became known, the US introduced a ban on all use of the drug in 1924 which is still in place today. In the UK, by contrast, the use of the drug as an analgesic became widely established in anaesthetic practice.

By the 1970s opioid receptors and the endogenous opioid peptides were being discovered. As outlined in Step 1.7, restrictive US opioid legislation was lifted in many states in the 1990s. Along with extensive and misleading drug marketing, an emphasis on pain as the ‘fifth vital sign’ and an assumption of correlating patient satisfaction with high quality of care, this led to the current epidemic of opioid prescribing not just in the US but across the developed world.

Timeline of the history of opioid use (a larger, screen-readable version is available to download below).