Analgesics | What Are Analgesics and How They Are Classified?

panadol (paracetamol)

What Are Analgesics

Analgesics, also known as painkillers, are a class of drugs that are generally used to reduce or relieve pain, an unlikable emotional and sensory experience in a human body linked with potential or actual tissue damage, or expressed in terms of such damage. The term “Analgesic” is derived from two Greek words – (1) an ("without") and (2) algos ("pain").

In other words, an analgesic is a medicinal agent, which relieves or reduces pain by heightening the threshold level in a body, without hampering consciousness or varying other sensory modalities. In short, therapeutic substances that diminish or reduce pain are termed as analgesics.

Classification of Analgesics

Drugs that are included in analgesics work in diverse ways to diminish or relieve pain. They act mainly on the central and peripheral nervous system. Narcotic drugs such as pathedine, synthetic drugs such as ketorolac, the non-steroidal anti-inflammatory drugs (NSAIDs) such as the salicylates (aspirin), and a variety of other drugs are included in analgesics. However, there are a few exceptions too. For example, tri-cyclic antidepressants and anticonvulsants are frequently used to treat neuropathic pain syndromes, but these drugs are not considered analgesics.

Based on the narcosis properties of the analgesic drugs, analgesics can be classified into the following groups.

Narcotic analgesics

The narcotic analgesics are the agents that cause sleep or loss of consciousness (narcosis) in conjunction with their analgesic effect. In other words, drugs that directly act on central nervous system (CNS) to relieve pain are termed as narcotic analgesics. In addition, the term narcotic becomes associated with the addictive properties of opioids and other CNS depressant agents.

The opiates and the derivatives of opiates (i.e., opioids) are the most frequently used narcotic analgesics. In the United States, these analgesics are also known as opioid analgesics (e.g., morphine, codeine, pethidine, etc.). Opioids are naturally occurring alkaloids and have been used for centuries for the management of pain symptoms. The earliest record of their use can be traced back to the ancient Egypt. However, they are also the most commonly used drugs of abuse.

The term “opioid” primarily refers to all compounds acting at the opioid receptors (e.g., mu, kappa, and delta), which are available both in the CNS and PNS (peripheral nervous system). These receptors are present spinally and supraspinally to mediate pain response. Opioids produce analgesia by acting on both presynaptic and postsynaptic nerve fibers. They presynaptically inhibit neurotransmitter release (e.g., substance P and glutamate) by blocking calcium channels while hyperpolarize cell membranes by opening potassium channels postsynaptically. All these actions eventually lead to the blocking of pain signals to the CNS.

Learn more about Weak and Strong Opioids.

WHO pain ladder

Non-narcotic analgesics

The non-narcotic (or non-opioid) analgesics act peripherally on the nervous system to reduce pain. Apart from their analgesic effect, the non-narcotic analgesics usually have two other properties: antipyretic and anti-inflammatory effects. Unlike narcotic analgesics, drugs of this class do not cause physical dependencies and narcosis. However, most of the drugs in this class are gastric irritant, except for pyrazolones and aniline-derivatives. For this reason, physicians generally recommend an antacid or anti-ulcerent when prescribing these drugs.

Non-narcotic analgesics can be broadly divided into 2 different classes: (1) non-acidic and (2) acidic analgesics.

(1) Non-acidic analgesics

Examples of these drugs include paracetamol (acetaminophen) and dipyrone (metamizole). Most often, these drugs are used in the management of mild to moderate pain. They are usually available as OTC (over the counter) medicine in most drug stores.

Metamizole, however, is often misclassified as a NSAID (non-steroidal anti-inflammatory drug), although it has a distinctly different profile of antipyretic action compared to NSAIDs. The drug also has spasmolytic properties.

Learn more about How the Nervous System Works.

aspirin (acetylsalicylic acid)

Acidic analgesics

This class of analgesics primarily includes the NSAIDs. Drugs of this class not only show chemical dissimilarities but also vary in their analgesic, antipyretic and anti-inflammatory properties. These drugs work principally by inhibiting the COX1 and COX2 enzymes, which help produce prostaglandins. However, they do not act on the lipooxygenase enzymes. NSAIDs can cause GI bleeding as they inhibit COX1, which protects the gastric mucosa from stomach acid. Their use is also associated with kidney problems and adverse cardiovascular events.

Aspirin (acetylsalicylic acid), the most widely used analgesic, is a prototype of this class. Other prominent NSAIDs include propionic acid, acetate acid and enolic acid derivates (e.g., naproxen, ibuprofen, diclofenac, meloxicam, etc.).

Published on: November 02, 2011
Last Updated: December 20, 2020

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