Substance P
The pain and inflammation neuropeptide that acts on NK1 receptors, given to people only as an experimental challenge
Substance P is a small neuropeptide released by sensory nerves and acting mainly on the NK1 receptor. It drives neurogenic inflammation and pain signalling. Infused into healthy adults it triggered headache in 71%; injected into skin it raises a wheal and flare. Blocking its receptor failed as a painkiller, though NK1 blockers are used against nausea.
What is it?
Substance P belongs to the neurokinins, small peptides spread through the brain, spinal cord and peripheral nerves. Its effects run through the NK1 receptor: in the brain it shapes stress responses and vomiting, in the spinal cord pain signalling. Only 5 to 7% of neurons in the relevant areas carry the receptor, and it is released in proportion to how intense and frequent a painful stimulus is.
Giving it to people is done to test ideas. In 21 healthy adults, a 20-minute intravenous infusion caused headache in 71% against 10% on placebo and widened the temporal artery, pointing to a role in headache. Injected into skin, it produced dose-dependent wheal and flare responses with no carry-over when repeated two weeks later, making it a useful challenge agent for testing drugs.
In seven healthy men, intravenous substance P raised luteinising hormone in a dose-dependent way without changing FSH or blood pressure.
Mast cells add a twist. In human tissue, including the dura around the brain, mast cells respond to substance P through a different receptor, MRGPRX2, not NK1, and the skin wheal from injected substance P is likewise mediated by MRGPRX2.
The drug story went the other way. NK1 antagonists eased sensitised pain in animals but failed in clinical trials across many pain conditions; aprepitant reached the market instead, as Emend, against nausea and vomiting. Substance P also reaches beyond the nerve: it stimulated aldosterone from adrenal adenoma tissue, an effect aprepitant blocked in culture.
As a measurement it is spreading into new uses. Salivary substance P separated infants who aspirate on swallow studies from those who do not, and Sjögren's disease from other dry-mouth conditions. In children after CAR-T therapy, a sharp rise marked severe cytokine release syndrome. These are small single-centre studies.
Mechanism of Action
Substance P is a neurokinin released from sensory and central nerves; it acts mainly through the NK1 receptor, and on mast cells through MRGPRX2.
Pain signalling
Released in proportion to the intensity of painful stimuli in the spinal cord; NK1 blockers nonetheless failed as painkillers.
Vomiting
NK1 signalling in the brain drives vomiting, which is why aprepitant works against chemotherapy nausea.
Mast cells and skin
Through MRGPRX2 it activates mast cells, causing the wheal and flare used as a skin challenge.
Published Research
These are the studies that describe substance P, read from their PubMed abstracts: three studies giving it to healthy people, a rat study and two reviews.
It provokes headache
21 healthy adults received substance P (1.5 pmol/kg/min for 20 minutes) or placebo, crossover. Headache followed in 71% against 10%, with greater intensity and a significant widening of the temporal artery.
A skin challenge
Intradermal substance P at 5 to 150 pmol produced dose-dependent wheal and flare responses in healthy volunteers, with no carry-over when repeated about two weeks later and no safety concerns.
A hormone signal
In seven healthy men, intravenous substance P at higher doses raised luteinising hormone in a dose-dependent way, while FSH, testosterone and blood pressure did not change.
Saliva and aspiration in infants
In 50 infants and toddlers, salivary substance P was 344.7 pg/mL in those who aspirated on a swallow study against 105.7 in those who did not; a 65 pg/mL cut-off gave 97% sensitivity.
Swallowing in neurological disease
In a pilot of 26 patients given nicergoline against 66 historical controls, swallowing scores improved and choking fell, alongside changes in substance P levels.
Saliva in Sjögren's disease
Salivary substance P was about three times higher in Sjögren's disease than in non-Sjögren dry mouth or controls, separating them with an AUC of 0.82.
Cytokine release after CAR-T
In children given CD19 CAR-T therapy, substance P rose far more in those who developed severe cytokine release syndrome, peaking at its onset before ferritin rose.
Shingles pain after nerve block
In herpes zoster, serum substance P and beta-endorphin fell after erector spinae plane blocks, though baseline levels did not differ from controls.
Liver cancer recurrence after transplant
Higher preoperative serum substance P was found in patients whose hepatocellular carcinoma recurred after liver transplantation, correlating with AFP and MELD scores.
Dengue
In a first serological study of dengue, NK1 receptor levels were lower in patients than controls; substance P did not differ by sex.
Arthritis pain in rats
In rats with chemically induced knee arthritis, blocking the NK1 receptor early reduced swelling, inflammatory signals in the joint and spinal cord, and the development of chronic pain.
Why blockers fail as painkillers
NK1 antagonists eased inflammation- and nerve-sensitised pain responses in animals but failed in clinical trials across many pain conditions, apparently blocking too little to relieve pain in people.
Neurobiology of substance P
Substance P acts through the NK1 receptor in mood, vomiting and spinal pain pathways; only 5 to 7% of neurons in those areas express the receptor, and release scales with stimulus intensity.
Aldosterone-producing adenomas
Substance P nerve fibres were present in 90% of human aldosterone-producing adenomas; substance P raised aldosterone secretion in 6 of 10 cultures, which aprepitant blocked in 3 of 4.
Mast cells use MRGPRX2
In mouse and human mast cells, including human dural mast cells, the receptor for substance P was MRGPRX2 rather than NK1, a finding relevant to headache.
Dosing Data
Substance P is not a treatment. Research infusions use up to 1.5 pmol/kg/min; intradermal challenge uses picomole doses.
Educational information, not medical advice. Each dose above is tagged with its source: an approved label, a published human study, animal data, or amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Side Effects & Contraindications
Reported Side Effects
From the challenge studies; it is never given as treatment.
Contraindications & Cautions
Not a treatment.
Legal Status
Substance P is not a medicine anywhere.
Frequently Asked Questions
?What is substance P?
A neuropeptide released by sensory nerves that drives pain signalling and neurogenic inflammation through the NK1 receptor.
?Does it cause headaches?
Infused into 21 healthy adults, it caused headache in 71% against 10% on placebo.
?Why are there no substance P painkillers?
Drugs blocking its receptor worked in animals but failed in clinical pain trials. One, aprepitant, is used for nausea.
?Is it sold as a supplement?
No product is approved, and it has only been given to people in research.
?Why does blocking NK1 not stop pain?
Trials of NK1 antagonists failed across many pain conditions, and the reason is not settled. Separately, mast cells, including those around the brain, respond to substance P through MRGPRX2 rather than NK1.
?Can substance P be measured in saliva?
Yes. Salivary levels distinguished infants who aspirate from those who do not, and Sjögren's disease from other dry-mouth conditions, in small studies.
References
- [pubmed] Al-Khazali HM, et al. "Effects of substance P on headache induction and arterial dilation in healthy adults." Cephalalgia, 2025;45(5):3331024251336132. PMID: 40369974.
- [pubmed] Ten Voorde W, et al. "Intradermal substance P as a challenge agent in healthy individuals." Clin Transl Sci, 2023;16(10):1856-1865. PMID: 37547990.
- [pubmed] Coiro V, et al. "Luteinizing hormone response to an intravenous infusion of substance P in normal men." Metabolism, 1992;41(7):689-91. PMID: 1377769.
- [pubmed] Kwon M, et al. "Substance P-neurokinin 1 receptor signal involves the development of osteoarthritis-induced chronic pain in rats." J Neuropathol Exp Neurol, 2025;84(10):879-891. PMID: 40613647.
- [pubmed] Hill R. "NK1 (substance P) receptor antagonists--why are they not analgesic in humans?." Trends Pharmacol Sci, 2000;21(7):244-6. PMID: 10871891.
- [pubmed] Mantyh PW. "Neurobiology of substance P and the NK1 receptor." J Clin Psychiatry, 2002;63 Suppl 11:6-10. PMID: 12562137.
- [pubmed] Steinhoff MS, et al. "Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease." Physiol Rev, 2014;94(1):265-301. PMID: 24382888.
- [fda-pi] EMEND (fosaprepitant) for injection Prescribing Information, section 1 (DailyMed version 20, effective February 20, 2026; read September 30, 2026).
- [other] US FDA, Drugs@FDA (openFDA): no application for substance P; EMEND (aprepitant), NDA 021549, Merck Sharp & Dohme. Read September 30, 2026.
- [pubmed] Duncan DR, et al. "Association Between Salivary Substance P and Aspiration on Videofluoroscopic Swallow Study in Infants and Toddlers." J Pediatr, 2026;297:115212. PMID: 42386123.
- [pubmed] Imsub J, et al. "Change in Swallowing Function and Substance P Levels Associated with Nicergoline in Neurological Disease: A Pilot Study." J Clin Med, 2026;15(12). PMID: 42355896.
- [pubmed] Rosso P, et al. "Substance P as a potential salivary biomarker to differentiate Sjögren's disease from non-Sjögren sicca syndrome: a pilot study." Front Immunol, 2026;17:1851367. PMID: 42344917.
- [pubmed] Molinos-Quintana Á, et al. "Temporal dynamics of substance P and its association with cytokine release syndrome after CD19 CAR-T therapy in pediatric B-ALL." Stem Cell Res Ther, 2026;17(1). PMID: 42265809.
- [pubmed] Lee JY, et al. "Reduction of Serum Substance P and Beta Endorphin in Herpes Zoster After Erector Spinae Plane Block." Pain Physician, 2026;29(6):E725-E732. PMID: 42804322.
- [pubmed] Li C, et al. "Predictive Value of Preoperative Serum Substance P Levels for Post-Transplant Recurrence in Patients With Hepatocellular Carcinoma." Transplant Proc, 2025;57(10):1964-1970. PMID: 41260946.
- [pubmed] Mehboob R, et al. "Substance P and NK-1R in Dengue: A First Serological Investigation." Health Sci Rep, 2026;9(8):e72820. PMID: 42517022.
- [pubmed] Lopez AG, et al. "Regulation of Aldosterone Secretion by Substance P and the Neurokinin Type 1 Receptor in Aldosterone-Producing Adenomas." J Am Heart Assoc, 2026;15(2):e045539. PMID: 41532541.
- [pubmed] Pellesi L, et al. "Substance P and PACAP activate dural mast cells through MRGPRX2, not NK1R or PAC(1)/VPAC receptors." Cephalalgia, 2026;46(8):3331024261480167. PMID: 42627518.
Sources & Citations
Studies were read from their PubMed abstracts; a preprint and an article under an expression of concern were not cited. Read October 7, 2026.
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