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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.

Key Points

  • Analgesics differ from anesthetics in their selective pain relief function.
  • They are broadly classified into narcotic and non-narcotic analgesics.
  • Anti-inflammatory analgesics, like aspirin and ibuprofen, reduce pain by minimizing inflammation.
  • Opioid analgesics, like morphine, act on the brain to relieve severe pain.
  • Understanding side effects and proper use of analgesics is essential for safe application.

Classification of Analgesics

Analgesics work in diverse ways to diminish or relieve pain. They act mainly on the central and peripheral nervous systems. Drugs commonly labeled as analgesics include:

  • Narcotic drugs like morphine, pethidine, etc.
  • Synthetic drugs like ketorolac, and
  • Non-steroidal anti-inflammatory drugs (NSAIDs) such as salicylates (aspirin)

There are some exceptions, though. Tricyclic antidepressants, for instance, and anticonvulsants, though frequently used to treat neuropathic pain syndromes, are not regarded as analgesics.

Analgesic Classification at a Glance

Classification Subclass Mechanism of Action Examples
Narcotic Analgesics Opiates and Opioids Act on the central nervous system (CNS) to relieve pain. Can cause sleep or loss of consciousness (narcosis). Morphine, Codeine, Pethidine, etc.
Non-narcotic Analgesics Non-acidic Analgesics Act peripherally to reduce pain. Generally used for mild to moderate pain. Paracetamol (Acetaminophen), Dipyrone (Metamizole)
Acidic Analgesics (NSAIDs) Inhibit COX1 and COX2 enzymes, reducing prostaglandin production. They have anti-inflammatory and antipyretic effects. Aspirin (Acetylsalicylic Acid), Ibuprofen, Naproxen, Diclofenac, Meloxicam

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 fact, the term narcotic often refers to the addictive agents like opioids and other CNS depressants.

What are opiates?

The opiates and their derivatives (i.e., opioids) are the most frequently used narcotic analgesics. In the United States, they are also known as opioid analgesics (↗️) (e.g., morphine, codeine, pethidine, etc.).

However, they are also the most commonly used drugs of abuse.

Overview and History

Opioids, including morphine and codeine, have been used for pain relief throughout history. The earliest record of their use can be traced back to the ancient Egypt. Opioids are naturally occurring alkaloids. They are derived from the opium poppy or synthesized chemically.

How opioids work?

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. They also hyperpolarize cell membranes by opening potassium channels postsynaptically. All these actions eventually lead to the blocking of pain signals to the CNS.

Types and Therapeutic Uses

  • Natural Alkaloids: Like morphine and codeine.
  • Synthetic Derivatives: Including methadone and meperidine.
  • Antagonists: Naloxone and naltrexone for overdose treatment.

Tolerance, Dependence, and Side Effects

Repeated use leads to tolerance and potential dependence. Withdrawal can induce severe symptoms.

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.

Origins and Development

Derived from compounds like salicylic acid, pyrazolone, and phenacetin, these drugs reduce pain by targeting inflammation.

Key Difference with Narcotic Analgesic

Unlike narcotic analgesics, non-narcotic drugs do not cause physical dependencies and narcosis. However, most non-narcotic drugs are gastric irritant, except for pyrazolones and aniline-derivatives. For this reason, physicians generally recommend an antacid or anti-ulcerent when prescribing these medications.

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)

(2) 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.

NSAIDs 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.).

Side Effects and Uses

They can cause hypersensitivity, peptic ulcers, and other specific toxicities like liver damage (acetaminophen) or renal issues (NSAIDs).

Glossary: Understanding Medical Terms

  • Analgesics: Medications that relieve or reduce pain.
  • Narcosis: A state of drowsiness or unconsciousness.
  • Central Nervous System (CNS): Part of the nervous system consisting of the brain and spinal cord.
  • Peripheral Nervous System (PNS): The part of the nervous system outside the brain and spinal cord.
  • Neurotransmitter Release: The process by which nerve cells communicate with each other or with muscles.
  • Opioids: A class of drugs that include natural, synthetic, and semi-synthetic compounds, commonly used for pain relief.
  • Prostaglandins: Chemicals in the body that contribute to inflammation, pain, and fever.
  • COX1 and COX2 Enzymes: Enzymes that help produce prostaglandins. Inhibition of these enzymes is the mechanism of action for many NSAIDs.
  • Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): A class of drugs that reduce inflammation and pain, not categorized as steroids.
  • Addiction: A condition of dependency on a substance, characterized by a compulsive need to use it.
  • Side Effects: Secondary, typically undesirable effects of a drug or medical treatment.


Understanding the types, mechanisms, and appropriate uses of analgesics is crucial in healthcare and pain management. While offering significant benefits, their side effects and potential for abuse necessitate cautious and informed usage.

First Published: November 02, 2011
Last Updated: March 16, 2024

⚠️ Disclaimer: The information provided in this article is for educational purposes only and should not be considered as medical advice. Please consult a healthcare professional for personalized advice.