Introduction
The universal challenge of chronic pain demands innovative, non-addictive solutions as healthcare systems worldwide grapple with opioid dependency.
Pain is a universal theme in illness, with its perception being highly subjective and often reported in both psychological and pathophysiological contexts1,2. The International Association for the Study of Pain (IASP) defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage"3. While often measured in severity, a fundamental classification of pain is by its duration, broadly classified as either acute or chronic. Acute pain has a sudden onset, is typically short-lived and disappears when the underlying cause is treated or healed. Conversely, chronic pain lasts or recurs for longer than three months, persisting after the original injury has healed2,3.
Chronic pain presents a major burden that is reported to impact more than 30% of people globally4. One in five adults in the US has chronic pain, while UK studies report prevalence rates of up to 43.5%5-7. The negative economic impact of this is severe, not only from a drug cost and medical expenses perspective, but also loss of productivity in the workplace8,9. Back pain, neck pain, osteoarthritis, and other musculoskeletal disorders rank among the top reasons individuals seek medical care for pain, which are leading causes of years lost to disability worldwide4.
While opioids were once commonly prescribed for chronic pain, the public health crisis of opioid use disorder (OUD) has augmented interest in developing non-opioid pharmacological alternatives for the management of chronic pain.
This paper will explore the non-opioid treatment and investment landscape for chronic pain, including emerging treatments, promising molecular targets and therapeutic approaches that are being leveraged for an opioid-free future.
Classification of Chronic Pain
Modern pain medicine recognizes distinct mechanisms underlying chronic pain, each requiring targeted therapeutic approaches for optimal patient outcomes.
In some chronic pain conditions, pain itself becomes a disease, a pathological process independent of an initial cause or origin. For instance, fibromyalgia is a chronic primary pain condition which is characterised by widespread musculoskeletal pain without a clear initiating injury10. In contrast, chronic post-surgical pain is a chronic secondary pain condition which persists beyond the healing process of an operation, and can often be traced back to tissue injury caused by the surgery itself11.
Until 2018, chronic pain conditions were not systematically represented in the WHO's International Classification of Diseases (ICD). This inadequate classification of chronic pain, merely confining it to being a symptom, was not ideal for the advancement of targeted therapies and the growth of pain medicine as a dedicated specialty12. It was responsible for limitations in epidemiological research, lack of effective health policy development and poor resource allocation. In June 2018, the IASP, in collaboration with the WHO addressed these shortcomings with the 11th revision of the ICD (ICD-11) in which systematic classification of chronic pain now distinguishes between chronic primary and chronic secondary pain syndromes, providing precise definitions and characteristic features for each diagnosis. It considers pain severity, its temporal course, and the influence of psychological and social factors12. This approach allows for a more nuanced and accurate diagnosis of chronic pain and facilitates better analgesic drug development through improved patient selection, more targeted research, and more accurate outcome measurements.
The Pathophysiology of Chronic Pain Involves Three Main Pathways
Understanding the complex neural pathways and cellular mechanisms that transform acute pain into persistent, debilitating chronic conditions.
Most pain signals start in nociceptive neurons located throughout the body, which relay these impulses to the spinal cord and up to the brainstem, completing the 'ascending pathway' that makes us aware of pain. In contrast, the 'descending pathway' from the brain modulates the perception of pain, typically acting to protect us by reducing pain signals when necessary, and interacting with the nervous, immune, endocrine, and autonomic systems to help restore physiological balance2,13.
In chronic pain, however, harmful pathophysiological adaptations occur in the central and peripheral nervous system (CNS and PNS respectively), including the formation of new neural pathways and pathology-specific brain alterations4,14. This can elicit modifications in the gene expression of proteins, receptors and neurotransmitters involved in pain signalling within the spinal cord and brain13. For instance, in diabetic neuropathy, abnormal nerve sprouts excessively produce substance P, a neuropeptide which activates nociceptive spinal neurons, and contributes to persistent pain4,15.
In terms of underlying causative mechanisms, chronic pain is broadly categorised into three types:
- Nociceptive pain arises from actual or threatened damage to non-neural tissue and is the most common form of chronic pain. It occurs in conditions like arthritis and most forms of spinal pain4.
- Neuropathic pain arises from injury or disease affecting neural tissue within the somatosensory nervous system, accounting for approximately 15-25% of chronic pain cases. It is commonly observed in conditions like diabetic neuropathy, postherpetic neuralgia and radiculopathy4,16.
- Nociplastic pain arises from altered nociception due to a sensitised CNS, despite no clear evidence of actual or threatened injury. It occurs in many primary chronic pain conditions such as fibromyalgia and complex regional pain syndrome (CRPS)17.
Some chronic pain conditions such as those involving cancer, may span multiple categories.
Nociceptive Pain | Neuropathic Pain | Nociplastic Pain | |
---|---|---|---|
Causes | Actual or potential damage to non-neural tissue | Injury or disease to neural tissue | CNS sensitisation without clear evidence of nerve or tissue damage |
Primary Site | Typically, PNS (with CNS processing) | PNS or CNS | CNS |
Chronic Pain Examples | Arthritic conditions e.g., rheumatoid arthritis, gout, osteoarthritis; Spinal pain e.g., neck and back pain; Inflammatory disorders e.g., tendonitis, bursitis | Peripheral neuropathic pain e.g., postherpetic neuralgia, diabetic neuropathy, trigeminal neuralgia, radiculopathy; Central neuropathic pain e.g., spinal cord injury | Widespread pain syndromes e.g., fibromyalgia; Visceral pain e.g., irritable bowel syndrome; Other chronic pain e.g., some tension-type headaches and non-specific back pain |
Common Descriptors | Throbbing, aching, pressure-like | Lancinating, shooting, electrical-like, stabbing | Similar to neuropathic pain. May also be described as diffuse, gnawing, aching, sharp |
Unfortunately, the early cellular changes that lead to chronic pain are often difficult to detect, making it harder to reverse or prevent the occurrence of chronic pain13. Understanding the tapestry of altered proteins, receptors and neurotransmitters involved in chronic pain offers hope for new targets in early diagnosis, prevention, and more effective treatments. However, present treatment regimens remain largely reactive rather than preventive, relying on analgesics which are broadly classified by therapeutic class into opioids and non-opioids.
Existing non-opioids
Current treatment options face significant limitations in efficacy and safety, highlighting the urgent need for innovative therapeutic approaches.
Opioids gained popularity during a time when there was little understanding of chronic pain mechanisms or the potential for opioid dependence and abuse18. Most clinically prescribed opioids bind to mu-opioid receptors, found in the CNS and peripheral tissues. They play a crucial role in mediating the effects of opioid drugs and endogenous opioid peptides, influencing pain relief, reward, and addictive behaviours. These receptors are classified as G protein-coupled receptors and are primarily activated by substances like morphine, leading to various physiological responses. While opioids binding to mu-opioid receptors trigger the brain's reward centres and effectively mask pain signals, repeated use causes long-lasting changes in the brain that lead to tolerance and dependence. As a result, higher doses are needed for the same therapeutic effect, and individuals on opioids ultimately become reliant on the drug for normal functioning19,20.
Existing non-opioid treatment options for chronic pain cut across various classes of drugs, as follows:
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Since their development in the mid-20th century, NSAIDs such as ibuprofen, naproxen and diclofenac have been among the most prescribed analgesics globally. They are useful for managing a certain degree of inflammation commonly present in chronic pain conditions like arthritis, complex regional pain syndrome and lower back pain13,21. NSAIDs' therapeutic effects are achieved through the selective or non-selective inhibition of cyclooxygenase (COX) enzymes, which in turn reduces prostaglandin (PG) synthesis. This leads to less inflammation and swelling and therefore, less pain at sites of injury. However, long-term use poses significant concerns especially related to gastrointestinal (GI) and cardiovascular (CV) side effects. Nonselective NSAIDs can damage the gut lining by inhibiting COX-1 and leading to ulcers, while COX-2 selective NSAIDs have a lower GI risk but may increase CV events by disrupting prostacyclin and thromboxane balance. Notably, some COX-2 selective NSAIDs like rofecoxib have been withdrawn from the market due to an increased risk of cardiovascular events such as heart attacks and strokes during long-term use13,22. Moreover, as NSAIDs mainly tackle inflammation that potentiates pain signals, they are less effective for neuropathic or more centralised chronic pain23.
Antidepressants
While not originally developed for chronic pain, antidepressants can alleviate chronic pain (especially neuropathic pain) in cases where depression is not a co-existing diagnosis13,24. For instance, amitriptyline, a tricyclic antidepressant has been widely used for the treatment of various chronic pain conditions including fibromyalgia and neuropathic pain. Another example is duloxetine, a serotonin-noradrenaline reuptake inhibitor (SNRI), which was the first FDA approved antidepressant for the treatment of diabetic neuropathic pain24. One proposed therapeutic mechanism is that some antidepressants modulate levels of serotonin and norepinephrine, thereby enhancing the activity of descending inhibitory pain pathways in the CNS, resulting in analgesic effects24.
But again, antidepressants tend to undergo extensive liver metabolism and have limited oral availability. For the management of chronic pain, adverse effects can range from mild symptoms like dry mouth to severe complications such as cardiac arrhythmias and serotonin syndrome. Serotonin syndrome is caused by excessive serotonin in the body and can lead to serious symptoms including hyperthermia and organ failure, and may be fatal if not treated promptly13,25.
Anticonvulsants
Originally designed to treat epileptic seizures, anticonvulsants have been repurposed for the treatment of chronic pain, especially neuropathic pain. Their 'nerve-calming' and analgesic properties are attributable to the blockade of sodium, potassium and calcium channels, enhancement of inhibitory neurotransmitters like Gamma-Aminobutyric Acid (GABA) and the inhibition of excitatory glutamate neurotransmission13,26. A good example is pregabalin, a GABA analogue and gabapentinoid, which was the first anti-epileptic to receive FDA-approval in 2004 for the treatment of painful diabetic neuropathy and postherpetic neuralgia. It exerts its analgesic effects by binding to the α₂δ subunit of voltage-gated calcium channels, thereby reducing the release of excitatory neurotransmitters27. However, variable efficacy, delayed onset of pain relief, dose-limiting side effects, and unwanted drug interactions are some concerns that have limited the widescale adoption and efficacy of anti-convulsants for the treatment of chronic pain13.
Some other non-opioid pharmacological options for chronic pain include acetaminophen, topical agents like lidocaine, NMDA receptor antagonists like ketamine in sub-anaesthetic doses, and muscle relaxants like baclofen28. Most therapies offer modest benefit with significant response variability and often require combinatorial approaches for adequate pain relief13,28. This disappointing landscape of non-opioid anti-pain options is fuelling the need for the development of novel non-opioid analgesics with improved safety and efficacy profiles.
Non-Opioid Class | Drug Name | Indication Labels |
---|---|---|
NSAIDs | Ibuprofen | Osteoarthritis, Rheumatoid Arthritis, Musculoskeletal Pain |
Celecoxib | ||
Diclofenac | ||
Naproxen | ||
Antidepressants | Amitriptyline | Fibromyalgia, Chronic Neuropathic Pain, Chronic Tension-type Headache |
Duloxetine | Fibromyalgia, Diabetic Peripheral Neuropathic Pain, Musculoskeletal Pain | |
Anticonvulsants | Gabapentin | Postherpetic Neuralgia |
Pregabalin | Postherpetic neuralgia, Diabetic Peripheral Neuropathy, Fibromyalgia, Spinal Cord Injury-associated Neuropathic Pain | |
Carbamazepine | Trigeminal Neuralgia |
Developing Effective Non-opioid Pain Solutions
Despite decades of research, breakthrough innovations are emerging with 154 clinical assets targeting novel mechanisms for safer, more effective chronic pain management.
Despite decades of research, the number of clinically validated molecular targets for analgesia remains limited. Most of the current non-opioid analgesics for chronic pain were originally developed for other indications, rather than being discovered through rational design for chronic pain. This dominance of 'repurposed therapeutics' for chronic pain highlights a key observation: target identification strategies have often failed to translate preclinical successes into effective clinical therapies in the pain space18.
From the current clinical-stage drug pipeline for chronic pain, assets targeting osteoarthritis pain, migraine, and diabetic neuropathic pain top the list, collectively accounting for about 40% (64) of the assets. Also, nearly 20% (26) of the current clinical-stage assets are in the late stages of clinical development.
For instance, after the transient receptor potential cation channel subfamily V member 1 (TRPV1) was cloned, there was great interest in developing small-molecule TRPV1 antagonists for pain relief, and indeed several leading drug candidates progressed to Phase I and II trials. These antagonists were expected to treat chronic pain by blocking TRPV1 mediated transmission of harmful stimuli from peripheral sensory neurons, consequently lowering pain perception. However, the enthusiasm in these assets waned due to significant adverse effects. Some candidates like AMG517 caused marked hyperthermia, while others, like MK-2295 obstructed heat pain perception, increasing the risk of burn injuries31.
Conversely, the wide-spread expression of mu-opioid receptors throughout both the peripheral and CNS is a strong contributor to their analgesic effect. This broad distribution allows opioids to provide relief, even when the specific underlying mechanisms are unclear or the root cause of the pain remains untreated. Moreover, the higher translatability of opioids from preclinical to clinical models may stem from the high conservation of opioid systems across vertebrate species18,33. Novel non-opioid analgesics that can provide effective treatment for chronic pain without targeting mu-opioid receptors, while minimising side effects and eliminating the risk of addiction, therefore seem to be the 'holy grail' in pain drug development.
A
Top Indications
B
Development Stage
C
Top Molecular Mechanisms
Figure 1:
Chronic pain pipeline as of July 2025, by Development Stage, Top Molecular Mechanism and Top Indication.
Source: GlobalData; Total assets analysed: 154. COX-1/2: Cyclooxygenase-1 and -2; Nav1.8: Voltage-gated sodium channel subtype 1.8; Nav1.7: Voltage-gated sodium channel subtype 1.7; NMDAR: N-methyl-D-aspartate receptor; CB1/CB2: Cannabinoid receptor type 1 and type 2.
Several clinical-stage pipeline assets target mechanisms that are distinct from existing non-opioids being prescribed for chronic pain. Among the top-targeted mechanisms identified are COX inhibitors and NMDAR antagonists such as ketamine, repurposed for pain management due to their ability to modulate excitatory neurotransmission. However, some of the mechanisms in the pipeline represent newer and emerging approaches to chronic pain management.