Substance P in Fibromyalgia: The Pain Peptide
Pain Neuropeptides
3-fold Higher in CSF
Cerebrospinal fluid substance P levels were 3-fold higher in fibromyalgia patients than normal controls in the landmark 1994 study that established the neuropeptide connection to widespread chronic pain.
Russell et al., Arthritis and Rheumatism, 1994
Russell et al., Arthritis and Rheumatism, 1994
View as imageFibromyalgia affects an estimated 2 to 4 percent of the global population with widespread pain that has no visible tissue damage to explain it. The discovery that substance P, an 11-amino-acid neuropeptide involved in pain transmission, is elevated 3-fold in the cerebrospinal fluid of fibromyalgia patients was one of the first biological findings to suggest the condition has a measurable neurochemical basis. Russell et al. demonstrated this in 1994 by measuring CSF substance P via radioimmunoassay in 32 fibromyalgia patients and 30 controls, finding levels significantly elevated at P less than 0.001.[1] That single finding launched decades of research into substance P as both a biomarker and therapeutic target for fibromyalgia. The story that unfolded is more complicated than the original discovery suggested: substance P is clearly elevated, but blocking it does not cure the pain. Understanding why requires examining how substance P fits within the larger neuropeptide landscape of chronic pain. For the broader picture of how multiple neuropeptides go wrong in chronic pain conditions, see Neuropeptide Dysregulation in Chronic Pain Syndromes and Peptide Biomarkers for Chronic Pain: What Blood Tests Might Reveal.
Key Takeaways
- Cerebrospinal fluid substance P levels were 3-fold higher in 32 fibromyalgia patients than 30 normal controls, with statistical significance at P less than 0.001 (Russell et al., Arthritis and Rheumatism, 1994)
- Substance P lowers the synaptic threshold in second-order spinal neurons and travels along the spinal cord to sensitize distant dorsal horn neurons, amplifying pain signals (Li et al., Neuroscience Research, 2015)
- NK-1 receptor antagonists failed to produce analgesia in clinical trials for chronic pain despite strong preclinical data, demonstrating that elevated substance P is not the sole driver of fibromyalgia pain (Zieglgansberger et al., Experimental Neurology, 2019)
- Substance P and CGRP are co-released from sensory nerve terminals, and their combined action drives neurogenic inflammation that sustains chronic pain states (Fan et al., Neural Plasticity, 2018)
- The substance P-NK1 receptor pathway is now recognized as a driver of neuroinflammation, immune modulation, and emotional processing beyond pain transmission alone (Kokabi et al., Neuropeptides, 2023)
- Reserpine-induced fibromyalgia models show that targeting downstream inflammatory cascades, rather than substance P directly, may be more therapeutically productive (Khodir et al., 2025)
What Is Substance P?
Substance P is an undecapeptide (11 amino acids: Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) belonging to the tachykinin neuropeptide family. It was first isolated from horse brain and intestine in 1931 by Ulf von Euler and John Gaddum, making it one of the oldest known neuropeptides. The "P" stands for "powder" or "preparation," not "pain," though the association with pain has become its most recognized function.
Substance P is synthesized from a precursor protein called preprotachykinin A (PPT-A) through alternative splicing that also produces neurokinin A. It is stored in dense-core vesicles within primary sensory neurons, particularly C-fibers (unmyelinated, slow pain fibers) and some A-delta fibers. When these neurons are activated by noxious stimuli, substance P is released from both central terminals in the spinal cord dorsal horn and peripheral terminals in tissues.
The primary receptor for substance P is the neurokinin-1 (NK-1) receptor, a G-protein-coupled receptor expressed on second-order neurons in the spinal cord, on immune cells, on vascular endothelial cells, and in brain regions including the amygdala and periaqueductal gray. Stander et al. reviewed the substance P-neurokinin receptor system in 2019, documenting its expression across multiple tissue types and its roles extending far beyond nociception into inflammation, mood regulation, and immune function.[4]
The Russell 1994 Discovery: Substance P Is Elevated in Fibromyalgia
The study that connected substance P to fibromyalgia was straightforward in design but profound in implications. Russell et al. collected cerebrospinal fluid via lumbar puncture from 32 patients meeting American College of Rheumatology criteria for fibromyalgia and 30 healthy controls matched for age and sex. CSF substance P was measured by radioimmunoassay.[1]
The result was unambiguous: substance P levels were approximately 3-fold higher in fibromyalgia patients. The elevation was statistically robust (P less than 0.001). However, CSF substance P levels correlated only weakly with the number of tender points found on physical examination. Russell et al. noted this disconnect explicitly: substance P is significantly elevated in FMS CSF, but other abnormalities must exist to more fully explain the symptoms.
This caveat proved prescient. The 3-fold elevation established substance P as a biomarker but not as the complete explanation.
Replication and Context
The Russell finding was not an isolated result. Multiple subsequent studies confirmed elevated CSF substance P in fibromyalgia, making it one of the most reproducible biological findings in a condition that has historically lacked objective biomarkers. The elevation was found to be relatively stable over time in individual patients, suggesting it reflects a trait abnormality rather than a transient state. Some studies found even higher ratios of elevation, though the approximately 3-fold figure remained the most commonly cited.
The clinical context matters: CSF substance P is not elevated in all chronic pain conditions. Patients with osteoarthritis, rheumatoid arthritis, and localized pain syndromes generally show normal CSF substance P levels. The elevation appears relatively specific to conditions characterized by central sensitization and widespread pain, including fibromyalgia and chronic widespread pain syndromes. This specificity strengthens the case that CSF substance P reflects a genuine central nervous system abnormality rather than a nonspecific response to chronic pain. The magnitude of the elevation also distinguishes fibromyalgia from conditions with borderline or inconsistent substance P changes.
Subsequent research revealed that fibromyalgia involves dysregulation of multiple neurotransmitter systems, including serotonin, norepinephrine, glutamate, and other neuropeptides, all operating within a framework of central sensitization.
How Substance P Amplifies Pain: Central Sensitization
Substance P contributes to pain amplification through several mechanisms that collectively produce central sensitization, the state in which the central nervous system amplifies normal sensory input into pain signals.
Spinal Dorsal Horn Sensitization
Li et al. detailed in 2015 how substance P acts within the spinal dorsal horn to amplify pain signaling. When released from C-fiber central terminals, substance P binds NK-1 receptors on second-order neurons in laminae I and II of the dorsal horn. This binding does not simply transmit a pain signal. It lowers the synaptic threshold of these neurons, making them respond to stimuli that would not normally activate them. Substance P can diffuse through spinal cord tissue and sensitize dorsal horn neurons at considerable distances from the original release site, a mechanism called volume transmission.[3]
This volume transmission explains a characteristic feature of fibromyalgia: widespread pain that is not confined to a single dermatome or nerve distribution. If substance P sensitizes dorsal horn neurons across multiple spinal segments, normal sensory input from large body areas could be interpreted as painful.
Co-release with CGRP
Substance P is not released alone. Fan et al. documented in 2018 that substance P and calcitonin gene-related peptide (CGRP) are co-released from sensory nerve terminals and act synergistically to produce neurogenic inflammation. Substance P causes plasma extravasation (fluid leakage from blood vessels), while CGRP produces vasodilation. Together, they create the tissue swelling, redness, and sensitization characteristic of neurogenic inflammation.[5]
The CGRP connection is clinically significant because CGRP-targeting monoclonal antibodies (erenumab, fremanezumab, galcanezumab) have proven effective for migraine prevention, another condition involving central sensitization and neuropeptide dysregulation. The success of anti-CGRP therapy for migraine raises the question of whether targeting the other half of the co-release, substance P, could work similarly for fibromyalgia. As discussed below, the answer has been disappointing. For more on CGRP-targeting approaches, the peptide antibody story is covered in the context of migraine therapeutics across the site.
Beyond Pain: Immune and Emotional Functions
Kokabi et al. reviewed the substance P-NK1 receptor pathway in 2023, documenting functions that extend well beyond nociception.[6] Substance P modulates immune cell activity, promoting pro-inflammatory cytokine release from macrophages and T cells. It affects mood and anxiety through NK-1 receptors in the amygdala and hippocampus. It influences gut motility through enteric nervous system signaling.
This pleiotropy helps explain why fibromyalgia patients often experience not just pain but also fatigue, cognitive dysfunction ("fibro fog"), mood disturbances, irritable bowel symptoms, and sleep disruption. If substance P is elevated systemically, its effects would not be limited to pain pathways alone. Esteban et al. reinforced this in 2021, reviewing how substance P-neurokinin signaling participates in neuroinflammation, immune regulation, and emotional processing as an integrated system rather than separate functions.[7]
Why NK-1 Receptor Antagonists Failed for Pain
The elevation of substance P in fibromyalgia CSF created an obvious therapeutic hypothesis: block the NK-1 receptor and the pain should decrease. Pharmaceutical companies invested heavily in developing NK-1 receptor antagonists for chronic pain in the 1990s and 2000s. Multiple compounds reached clinical trials. All failed to demonstrate analgesic efficacy.
Zieglgansberger et al. analyzed this failure in their 2019 review of substance P and pain.[2] Several factors explain why blocking a clearly elevated pain-related neuropeptide did not reduce pain:
Redundancy in pain signaling. Substance P is one of many neurotransmitters released at spinal synapses. Glutamate, brain-derived neurotrophic factor (BDNF), and other neuropeptides also transmit pain signals. Blocking one transmitter leaves the others intact. Pain is multiply encoded.
Substance P's role may be modulatory, not transmissive. Substance P does not carry the acute pain signal. That function belongs primarily to glutamate acting on AMPA and NMDA receptors. Substance P modulates the gain of the system, turning up sensitivity over time. Blocking it may prevent future sensitization without reversing existing sensitization.
Central vs. peripheral action. Many NK-1 antagonists tested had limited CNS penetration. Since the relevant substance P elevation is in the cerebrospinal fluid and spinal cord, peripherally acting antagonists would miss the target.
Timing of intervention. By the time patients present with established fibromyalgia, central sensitization may be self-sustaining through mechanisms that no longer depend on substance P input. Blocking substance P in an already sensitized system is like removing the match after the fire is burning. Structural changes in dorsal horn neurons, including altered gene expression and synaptic remodeling, may persist independently of the neuropeptide signals that initiated them.
The failure of NK-1 antagonists for pain contrasts sharply with their success in other indications. Aprepitant (Emend) is an FDA-approved NK-1 antagonist for chemotherapy-induced nausea and vomiting, and NK-1 antagonists have shown efficacy in clinical trials for depression and anxiety. Kepler et al. studied NK-1 receptor antagonist effects on pain and inflammation in 2015, finding that the receptor has context-dependent functions that vary by tissue type and disease state.[8]
Substance P in the Broader Fibromyalgia Neurochemistry
Substance P elevation does not exist in isolation in fibromyalgia. The condition involves a constellation of neurochemical abnormalities:
| System | Finding in Fibromyalgia | Relationship to Substance P |
|---|---|---|
| Serotonin | Decreased in CSF and serum | Serotonin normally inhibits substance P release |
| Norepinephrine | Decreased descending inhibition | Reduced pain suppression amplifies SP effects |
| Glutamate | Elevated in insular cortex | Co-transmitter with SP at dorsal horn synapses |
| BDNF | Elevated in serum | Promotes central sensitization alongside SP |
| Nerve Growth Factor | Elevated in CSF | Increases substance P synthesis in DRG neurons |
| Endorphins | Variable findings | Endogenous opioid compensation may be inadequate |
Lisowska et al. reviewed substance P's role in chronic pain conditions in 2015, placing the fibromyalgia findings within the broader context of chronic pain neurochemistry.[9] The low serotonin and norepinephrine findings are particularly relevant because these neurotransmitters normally suppress substance P release from primary afferent terminals. When they are deficient, substance P release becomes disinhibited, potentially explaining the elevation.
This insight helps explain why duloxetine (a serotonin-norepinephrine reuptake inhibitor) is one of three FDA-approved drugs for fibromyalgia. By increasing serotonin and norepinephrine in the spinal cord, duloxetine may indirectly reduce substance P release from primary afferent terminals, addressing the upstream cause rather than blocking the downstream receptor. For how opioid peptides interact with substance P in pain modulation, see Beta-Endorphin: The Runner's High Peptide and Dynorphin and the Kappa Receptor: The "Dark Side" of Opioid Peptides.
Animal Models and New Therapeutic Directions
Khodir et al. investigated a reserpine-induced fibromyalgia model in 2025, demonstrating that honokiol ameliorated fibromyalgia-like symptoms in rats through anti-inflammatory and antioxidant mechanisms rather than direct substance P antagonism.[10] This approach reflects a broader shift in thinking: rather than targeting substance P directly, targeting the neuroinflammatory cascade that substance P helps maintain may be more therapeutically productive.
The substance P-NK1 receptor pathway continues to generate research across multiple disease contexts. Rong et al. reviewed the latest evidence in 2025, documenting ongoing clinical interest in NK-1 receptor modulation for conditions ranging from chronic pain and depression to inflammatory bowel disease and pruritus.[11] The golestaneh et al. 2022 review examined how the substance P-NK1 pathway participates in neurological diseases more broadly, including neurodegeneration and traumatic brain injury, suggesting that the neuropeptide's pathological role extends well beyond pain sensitization.[12]
Substance P as a Biomarker: Clinical Utility
Despite the therapeutic failure of NK-1 antagonists for pain, CSF substance P retains potential value as a biomarker for fibromyalgia and central sensitization states.
Diagnostic potential. Fibromyalgia diagnosis remains clinical, based on symptom questionnaires and physical examination. An objective biomarker would strengthen diagnostic confidence and reduce the skepticism that many patients encounter. CSF substance P elevation is consistent and reproducible across studies.
Treatment monitoring. If substance P levels decrease with effective treatment, CSF measurement could serve as an objective outcome measure for clinical trials. Pregabalin (Lyrica), another FDA-approved fibromyalgia drug, has been studied for its effects on CSF substance P levels.
Subtyping. Not all fibromyalgia patients may have the same neurochemical profile. Substance P levels could help identify a subgroup of patients with predominantly neuropeptide-driven central sensitization, who might respond differently to targeted therapies than patients with other predominant mechanisms.
Practical barriers. Lumbar puncture is invasive, painful, and expensive. It requires physician expertise and carries small but real risks (post-dural puncture headache, infection). Using CSF substance P as a routine clinical biomarker faces the fundamental barrier that the measurement method is more burdensome than the condition it aims to diagnose. Blood-based substance P measurements do not reliably reflect CNS levels. For the state of peptide biomarker development across pain conditions, see Peptide Biomarkers for Chronic Pain: What Blood Tests Might Reveal.
Evidence Gaps and Open Questions
Why do NK-1 antagonists work for nausea but not pain? The most puzzling aspect of substance P pharmacology is the dissociation between its clear role in pain sensitization and the failure of its antagonists to produce analgesia. Understanding this dissociation fully could unlock new therapeutic strategies.
Peripheral substance P in fibromyalgia. Most research focuses on CSF levels. Whether substance P is also elevated in peripheral tissues (skin, muscle, fascia) of fibromyalgia patients, and whether peripheral elevation contributes to symptoms, is incompletely studied.
Substance P in fibromyalgia subtypes. Fibromyalgia is increasingly recognized as a heterogeneous condition. Whether substance P elevation characterizes all fibromyalgia patients or defines a specific subtype has not been determined.
Combination targeting. Given the redundancy of pain signaling systems, targeting substance P alongside other pain mediators (glutamate, CGRP, NGF) simultaneously might succeed where single-target approaches failed. No combination trial has tested this hypothesis in fibromyalgia.
Sex differences. Fibromyalgia disproportionately affects women (approximately 7:1 female to male ratio). Whether substance P elevation differs by sex, and whether sex hormones influence substance P metabolism or NK-1 receptor expression, are open questions with limited data. For the broader context of how neuropeptides relate to other pain conditions, see Peptide Approaches to Neuropathic Pain: Targeting the Source and Neuropeptide Y: The Stress Resilience Peptide.
The Bottom Line
Substance P, an 11-amino-acid neuropeptide involved in pain transmission, is elevated approximately 3-fold in the cerebrospinal fluid of fibromyalgia patients. This elevation, first demonstrated by Russell et al. in 1994, was one of the earliest objective biological findings in a condition long dismissed as subjective. Substance P amplifies pain through dorsal horn sensitization, volume transmission across spinal segments, and synergistic neurogenic inflammation with CGRP. However, NK-1 receptor antagonists that block substance P signaling failed to produce analgesia in clinical trials, revealing that substance P is a contributing factor but not the sole driver of fibromyalgia pain. The neuropeptide's roles extend beyond nociception into immune modulation, mood regulation, and gut function, helping explain fibromyalgia's multisystem symptom profile. Current research focuses on targeting the neuroinflammatory cascade that substance P helps maintain rather than blocking the peptide directly.