opioid

opioid, class of drugs derived from substances that occur naturally in the opium poppy (Papaver somniferum) and that are widely used for pain relief and sedation. Opioid drugs include prescription pain relievers, such as fentanyl, oxycodone, and hydrocodone, and illegal substances, such as heroin and illicitly manufactured fentanyl. These drugs act on molecules known as opioid receptors in the brain to produce a variety of effects, including pain relief and euphoria. Their actions render them not only highly effective in reducing pain but also highly addictive. Addiction to opioids is referred to medically as opioid use disorder.

Classification

The term opioid is sometimes used interchangeably with opiate; the latter, however, refers more narrowly to natural opioids—namely codeine, heroin, morphine, and opium. Opioids and opiates are classified as narcotics, which are any drugs that produce pain relief and narcosis (a state of stupor or sleep) and that can cause addiction. Today the term narcotic is used almost exclusively in reference to opioids and opiates. Opioids may be classified further as agonists, partial agonists, or antagonists, according to whether they respectively activate, partially activate, or block opioid receptors, and they may be described as alkaloids or as semisynthetic or synthetic compounds, depending on whether they are in their natural form (alkaloid) or have been partially or fully synthesized. In the United States, narcotics are regulated under the Controlled Substances Act, which established a classification system with five schedules to identify drugs based on their potential for abuse, their applications in medicine, and their likelihood of producing dependence. The act is implemented by the Drug Enforcement Administration, which is empowered to prosecute violators of laws governing these controlled substances.

Historical perspective

Cultivation of the opium poppy for its active constituents is thought to have begun about 3000 bce, and opioids that occur naturally in the plant have been used specifically for relieving pain and for producing euphoria since at least the time of the ancient Greeks. Extracts of opium poppy were smoked, eaten, or otherwise consumed in the form of laudanum (a mixture of alcohol and opium). In 16th- and 17th-century Europe laudanum was used to treat a variety of ailments and was commonly used to treat pain and to sedate patients. In the first half of the 19th century, the pharmacologically active components of opium were isolated and characterized. The first compound to be isolated was morphine, by German pharmacist F.W.A. Sertürner about 1804. Codeine was isolated from morphine in 1832 and was found to produce much milder effects. The chemical formula of morphine was identified in 1847.

With the invention of the hollow hypodermic needle in 1853, morphine use became more widespread. Administration of the drug by injection was more effective than taking the same amount of the drug orally. However, the availability of morphine injections fueled abuse of the drug. In 1890 the U.S. Congress introduced a tax on opium and morphine, and in 1909 U.S. lawmakers banned opium imports. Meanwhile, however, in 1898 the German chemical and pharmaceutical company Bayer developed heroin from morphine. Heroin is 5 to 10 times as potent as morphine, and it quickly became the drug of choice for opium addicts. The addictive nature of morphine inspired researchers to develop safer synthetic substitutes, which led to the generation of opioids such as meperidine, methadone, and levorphanol.

Mechanism of action and effects

The pain-relieving effects of opioids are mediated by receptors located in the central nervous system and the peripheral nervous system. Unfortunately, the actions of opioids at opioid receptors in the brain that mediate neural pathways involved in pain relief and euphoria and that mediate so-called reward pathways that trigger feelings of pleasure also cause opioids to be extremely addictive. Over time, persons who take opioids, even when prescribed by a doctor, are at risk of developing tolerance, physical dependence, and opioid use disorder. The steady progression from tolerance to dependence and addiction, in which progressively larger doses must be used to achieve the same pleasurable effects, leaves individuals at risk of overdose. Once the drug wears off, users must endure symptoms of physical and psychological withdrawal.

An opioid overdose can severely depress the central nervous system, resulting in respiratory failure and death. When an overdose occurs, the most effective treatment is the drug naloxone, which can relieve respiratory depression. Naloxone is used worldwide in medical emergencies involving opioid overdose and can be administered intravenously, intramuscularly, or subcutaneously; it is also available as a nasal spray.