Substance P

SP; a tachykinin, encoded by the TAC1 gene (preprotachykinin A) alongside neurokinin A; its receptor is NK1 (TACR1)

Cognitive & neuro

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Summary

Class: Naturally occurring undecapeptide (11 amino acids) of the tachykinin family
Mechanism: Activates the NK1 receptor to transmit pain signals in the spinal cord, trigger neurogenic inflammation in tissue, dilate vessels and drive the vomiting reflex in the brainstem
Routes studied: Intradermal and intra-arterial injection in human physiology experiments; intrathecal and central routes in animals. There is no therapeutic exogenous use of substance P
Half-life: Seconds to a few minutes; rapidly degraded by ACE and neutral endopeptidase
Storage: Research peptide; lyophilised, cold storage. Not a consumer product
Regulatory status: The peptide is not approved anywhere. Its receptor antagonists are approved medicines — see below

Doses are as reported in studies or by the community — never recommendations.

1What it is

Substance P is one of the oldest named bioactive peptides: von Euler and Gaddum described an unidentified active "substance" in brain and gut extracts in 1931 (the P stood for powder), and it took forty years to sequence it.

It is your own peptide, and its job is to signal that something is wrong. Released from the central endings of sensory nerves, it transmits pain — particularly sustained, inflammatory pain — to the spinal cord. Released from the peripheral endings of those same nerves, it dilates local vessels, makes them leaky and recruits mast cells, producing the redness, swelling and itch of neurogenic inflammation. In the brainstem it drives vomiting. In the brain more broadly it has been linked to mood and stress.

Crucially, this is a profile about a molecule you do not want more of. Every successful drug in this space is an NK1 antagonist. Nobody administers substance P therapeutically, and nobody self-administers it. Community use: essentially none documented, and the pharmacology gives a clear reason why — exogenous substance P produces pain, flare and itch.

2How it's thought to work

Binds NK1, a Gq-coupled receptor, raising intracellular calcium; neurokinin A and B prefer NK2 and NK3.

Spinal nociception. released from C-fibre terminals in the dorsal horn, contributing to central sensitisation and wind-up. NK1-expressing dorsal horn neurons are essential for some forms of hyperalgesia.

Neurogenic inflammation. peripheral release causes vasodilation, plasma extravasation and mast-cell degranulation, with histamine release amplifying itch.

Emesis. NK1 receptors in the nucleus tractus solitarius and area postrema mediate the delayed phase of chemotherapy-induced vomiting — the basis of the approved antagonists.

Immune modulation. NK1 receptors on macrophages, lymphocytes and mast cells link the nervous and immune systems.

Central. implicated in stress responses, anxiety and depression, which drove an enormous and ultimately disappointing drug programme.

Degradation by ACE. means ACE inhibitors raise local substance P, which is the leading explanation for ACE-inhibitor cough and part of the angioedema story.

Substance P released from sensory nerve endings onto the NK1 receptor. Vessels beside the ending widen and leak and surrounding nerves are sensitised — neurogenic inflammation — while the pain signal travels on centrally.Schematic — not to scale

3What the research shows

NK1 antagonists for chemotherapy-induced nausea and vomiting

The one unambiguous clinical success of this pathway. Aprepitant, its intravenous prodrug fosaprepitant, rolapitant and netupitant (combined with palonosetron as NEPA) are approved and are standard components of antiemetic regimens for highly emetogenic chemotherapy, added to a 5-HT3 antagonist and dexamethasone. Large randomised trials established improved complete-response rates, particularly in the delayed phase (Hesketh 2003, J Clin Oncol; Poli-Bigelli 2003, Cancer; and the registration programmes for the later agents). [3]

Graded A (negative)

NK1 antagonists for depression and anxiety

An early Phase 2 result with aprepitant in major depression (Kramer 1998, Science) generated enormous excitement. It did not replicate: multiple large randomised trials of aprepitant and other NK1 antagonists in major depressive disorder failed, and the indication was abandoned. This is one of the most cited cautionary tales in CNS drug development. [4]

Graded A (negative)

NK1 antagonists for pain

Despite an extremely strong preclinical case for substance P in nociception, NK1 antagonists consistently failed as analgesics in human trials across multiple pain conditions. The disconnect between the animal pain literature and the human trials remains incompletely explained — redundancy of nociceptive signalling is the usual explanation.

Graded A/B (mostly negative)

NK1 antagonists for chronic pruritus

Serlopitant and aprepitant showed promise in small studies of chronic itch and prurigo nodularis, but serlopitant's Phase 3 programme in prurigo nodularis and chronic pruritus did not meet its endpoints and development stopped †.

Human substance P physiology

Intradermal injection reproducibly produces a wheal-and-flare response; intra-arterial infusion produces vasodilation. These are controlled human experiments used to study neurogenic inflammation, not treatments.

Graded A (by proxy)

Capsaicin and substance P depletion

Topical high-concentration capsaicin (approved for neuropathic pain) works in part by defunctionalising substance-P-containing C-fibres — an approved therapy that removes substance P signalling rather than adding it.

Graded C/D

Wound healing, arthritis, IBD, migraine

Substance P appears in animal models of all of these. Nothing has produced a human treatment based on giving the peptide.

4Reported side effects and interactions

Literature-reported

Literature (the peptide). Exogenous substance P produces pain, burning, itch, flare, vasodilation and, systemically, hypotension and flushing. In other words its side effects are its actions.

Literature (NK1 antagonists, for context). Aprepitant and related drugs cause fatigue, hiccups, constipation and headache; aprepitant is a moderate CYP3A4 inhibitor and interacts with dexamethasone, certain chemotherapies, warfarin and hormonal contraceptives — a genuinely important interaction set. Rolapitant inhibits CYP2D6.

Community-reported

None for the peptide. Not in self-directed use.

Theoretical. Any sustained increase in substance P signalling would be expected to increase pain, itch and inflammation. There is no plausible rationale for exogenous administration outside a laboratory experiment.

Interactions. ACE inhibitors and neprilysin inhibitors (sacubitril) reduce substance P degradation, raising local levels — the mechanism behind ACE-inhibitor cough and a contributor to angioedema risk.

Markers researchers commonly monitor

There is no protocol for administering this peptide outside experimental physiology. In human experimental and clinical work the relevant measures are:

Marker	Why it's watched
Local flare area and wheal diameter	the standard quantitative readout of intradermal substance P, measured by laser Doppler or planimetry.
Blood pressure	systemic substance P is a vasodilator.
Validated pain and itch scales (NRS, VAS, 5-D itch)	the endpoints in the NK1-antagonist trials.
Plasma or tissue substance P	research assays; levels are reported as altered in fibromyalgia and some chronic pain states, but the assays are unreliable and this is not a clinical test.
For anyone taking an NK1 antagonist clinically: no routine blood monitoring is mandated, but CYP3A4 interactions make a medication review the relevant safety step

Free for everyone. Safety information is never behind a paywall.

5Reconstitution and handling

Not applicable as a consumer practice. Research-grade substance P is prepared in sterile solution for intradermal or intra-arterial administration in human experimental physiology, in quantities small enough to produce a local response, and is degraded within minutes. There is no vial format, no community practice, and no calculator preset. For the approved NK1 antagonists, formulation and handling follow their product labels (oral capsules, oral suspension, IV infusion), and those are prescription medicines rather than research compounds.

6Commonly studied alongside

CGRP (#159 — the other principal sensory neuropeptide, co-released from the same nerve fibres, with the opposite drug-development story), capsaicin/TRPV1 agents, 5-HT3 antagonists in the antiemetic setting.

Informational cross-links. Not stack recommendations.

7Regulatory status by region

Region	Status (checked —)
United States	The peptide: not FDA-approved, research use only. NK1 antagonists aprepitant (Emend), fosaprepitant, rolapitant (Varubi) and netupitant/palonosetron (Akynzeo) are FDA-approved for chemotherapy-induced nausea and vomiting
European Union	Same pattern: peptide not authorised; aprepitant, fosaprepitant and netupitant/palonosetron are EMA-authorised. Rolapitant's EU status should be checked †
United Kingdom	Peptide not licensed; NK1 antagonists MHRA-licensed, prescription-only
Australia	Peptide not approved; aprepitant and fosaprepitant TGA-registered
South Africa	Peptide not registered; aprepitant availability should be confirmed with SAHPRA †
Sport	Substance P is not listed by WADA. NK1 antagonists are not prohibited. As an unapproved peptide, exogenous substance P would fall under S0 if used

8Open questions

Why did such a strong preclinical case for substance P in pain fail so completely in human analgesic trials?
Does the NK1 pathway have a real role in chronic itch, given the mixed Phase 2 and negative Phase 3 results?
What explains the one positive depression study that never replicated?
Is raised substance P in conditions like fibromyalgia causal or incidental?

9References

1von Euler US, Gaddum JH (1931). An unidentified depressor substance in certain tissue extracts. J Physiol. PubMed
2Chang MM, Leeman SE, Niall HD (1971). Amino-acid sequence of substance P. Nat New Biol. PubMed · DOI
3Hesketh PJ et al. (2003). The oral neurokinin-1 antagonist aprepitant for the prevention of chemotherapy-induced nausea and vomiting. J Clin Oncol. PubMed
4Kramer MS et al. (1998). Distinct mechanism for antidepressant activity by blockade of central substance P receptors. Science. PubMed · DOI
5Keller M et al. (2006). Lack of efficacy of the substance P (neurokinin-1 receptor) antagonist aprepitant in the treatment of major depressive disorder. Biol Psychiatry. PubMed · DOI
6Hill R (2000). NK1 (substance P) receptor antagonists — why are they not analgesic in humans? Trends Pharmacol Sci. PubMed · DOI
7Mashaghi A et al. (2016). Neuropeptide substance P and the immune response. Cell Mol Life Sci. PubMed · DOI

References are being resolved to PubMed/DOI identifiers; links search PubMed for the citation until then.

† Being confirmed by our reviewers against current sources.

10Community

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11Page history

30 Oct 2026Profile published — awaiting medical review

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