VIP

VIP, short for vasoactive intestinal peptide (also written vasoactive intestinal polypeptide), is a 28-amino-acid neuropeptide produced naturally by the human body. It was first isolated from intestinal extracts in 1970, and the name captures what researchers saw first: a substance from the intestine that dilated blood vessels. Decades of work since then have shown that the intestine is only one of many places where VIP matters.

Chemically, VIP has the sequence HSDAVFTDNYTRLRKQMAVKKYLNSILN with an amidated C-terminus, a molecular formula of C₁₄₇H₂₃₈N₄₄O₄₂S and a molecular mass of roughly 3326 g/mol. It is unglycosylated, highly basic, and belongs to the secretin/glucagon superfamily of peptides. Its closest structural relative is PACAP (pituitary adenylate cyclase-activating polypeptide), with which it shares receptors and many biological effects. The human VIP gene also encodes a second bioactive fragment, PHM-27, from the same precursor protein.

Unlike many compounds discussed in peptide circles, VIP is not a laboratory invention. It is an endogenous signalling molecule found in neurons of the central and peripheral nervous systems, in the enteric nervous system of the gut wall, in lymphoid tissue, in the lung, and in the pancreas. Human VIP, porcine VIP and rat VIP are identical in sequence, which is why so much of the animal literature translates directly in mechanistic terms.

The synthetic pharmaceutical form of VIP has its own international nonproprietary name: aviptadil. When you read about VIP in a clinical trial register or a regulatory document, aviptadil is almost always the molecule being discussed. Keeping the two names linked is useful, because searching only for "VIP peptide" misses most of the serious human research.

This guide covers what is established, what is plausible but unproven, and what is being claimed well ahead of the evidence. It is written for educational purposes only and is not a protocol.

VIP acts through three class B G-protein-coupled receptors: VPAC1, VPAC2 and PAC1. VIP binds VPAC1 and VPAC2 with high affinity and PAC1 with much lower affinity, whereas PACAP binds all three readily. Receptor binding activates the Gs protein, which stimulates adenylyl cyclase and raises intracellular cAMP, with downstream signalling largely through protein kinase A. Secondary pathways involving phospholipase C and protein kinase C have also been described in specific cell types.

Receptor distribution explains much of VIP's apparent versatility. VPAC1 predominates in the lung, the intestinal epithelium, the liver and on many immune cells. VPAC2 is enriched in smooth muscle, pancreatic beta cells and the suprachiasmatic nucleus of the hypothalamus. Because a single ligand reaches receptors in vascular, epithelial, neural, endocrine and immune tissue, VIP produces a broad pattern of effects rather than one clean pharmacological action. That breadth is scientifically interesting and, for drug development, a liability: selectivity is hard to achieve.

Structure-activity work using alanine scanning and photoaffinity labelling has mapped which residues matter for VPAC1 versus VPAC2 binding, and selective agonists have been designed from that information. This line of research is the basis for attempts to capture one effect of VIP, for example VPAC2-mediated insulin secretion, without triggering the whole cascade.

The central pharmacokinetic fact about VIP is its instability. Reported plasma half-life is on the order of one to two minutes, driven by rapid enzymatic degradation and renal clearance. A molecule that disappears that fast cannot be dosed conveniently, which is why clinical work has relied on continuous intravenous infusion, local injection, inhalation, or experimental carriers such as liposomal and sustained-release formulations. Anyone evaluating claims about VIP should ask first how the peptide was delivered, because route and formulation drive almost everything about its practical effects.

Vasodilation. VIP is a potent dilator of both systemic and pulmonary vessels. It relaxes vascular smooth muscle through cAMP-dependent signalling, lowers arterial pressure when given systemically, and contributes to local blood flow regulation in tissues where VIP-containing nerve fibres are dense. This vascular action is the single most consistently reproduced property of the peptide and the reason it was investigated in pulmonary hypertension.

Airways. VIP relaxes bronchial smooth muscle, acting as an endogenous bronchodilator, and stimulates fluid secretion from airway submucosal glands. In nonclinical work it has also been reported to support surfactant production by alveolar type II cells, which is the rationale that carried aviptadil into acute respiratory distress research.

Gastrointestinal function. In the gut, VIP is a principal non-adrenergic, non-cholinergic inhibitory neurotransmitter of the enteric nervous system. It relaxes smooth muscle, mediates descending relaxation during peristalsis, increases mucosal blood flow, and stimulates active water and electrolyte secretion by the intestinal epithelium through adenylyl cyclase activation. Research also points to roles in epithelial differentiation and barrier integrity, topics covered more broadly in our overview of peptides studied for gut health.

Endocrine signalling. VIP potentiates glucose-dependent insulin secretion from pancreatic beta cells, an effect attributed largely to VPAC2 and mediated through ion channel modulation. This has motivated interest in VPAC2-selective agonists for type 2 diabetes, though no such agent has reached approval.

Circadian timing. One of the best-characterized roles of VIP is in the master clock. In mice lacking the VPAC2 receptor, the suprachiasmatic nuclei fail to sustain normal circadian rhythms of rest and activity, fail to show circadian expression of core clock genes including mPer1, mPer2 and mCry1, and fail to show the acute light-induced induction of mPer1 and mPer2 seen in normal animals (Harmar et al., Cell, 2002). VIP signalling through VPAC2 is what keeps thousands of individual clock neurons oscillating in phase with one another.

VIP is released not only from neurons but also from activated T cells, and receptors for it are present on macrophages, dendritic cells, microglia and lymphocytes. This makes it one of the clearest examples of neuroendocrine-immune crosstalk. The comprehensive review by Delgado, Pozo and Ganea in Pharmacological Reviews (2004) traced VIP's transition from gut hormone to candidate immune modulator and summarized the cellular and molecular mechanisms involved.

In innate immunity, VIP and PACAP broadly suppress the production of pro-inflammatory cytokines and chemokines by macrophages, microglia and dendritic cells. In adaptive immunity, VIP has been reported to shift the Th1/Th2 balance toward Th2 responses, to generate tolerogenic dendritic cells, and to favour the induction and recruitment of regulatory T cells. The overall direction of effect in these models is anti-inflammatory and tolerance-promoting.

Animal models of inflammatory disease have repeatedly shown benefit. VIP has been studied in collagen-induced arthritis, in experimental colitis, and in models of sepsis, and preclinical work in Citrobacter rodentium-induced colitis showed that VIP treatment prevented the infection-associated rise in paracellular permeability and prevented the mislocalization of tight junction proteins including zonula occludens-1, occludin and claudin-3 (Conlin et al., 2009).

The important caveat is that these are animal and cell-culture findings. Promising immunomodulation in a mouse does not establish benefit in a human patient, and VIP's short half-life plus its potent vascular effects make systemic anti-inflammatory dosing in people genuinely difficult. No regulator has approved VIP or aviptadil for any inflammatory or autoimmune condition. If you are interested in how this class of research is weighed more generally, see our review of peptides studied for inflammation.

This section describes research findings, not treatment options. Nothing here should be read as a recommendation to use VIP for an inflammatory condition.

Pulmonary hypertension. The most-cited early human work is Petkov and colleagues in the Journal of Clinical Investigation (2003). Starting from the observation that VIP appeared deficient in the serum and lung tissue of patients with primary pulmonary hypertension, the investigators administered inhaled VIP to eight patients. They reported a decrease in mean pulmonary artery pressure along with increases in cardiac output and mixed venous oxygen saturation. This was a small, open study in a single group of patients, and it has not translated into an approved indication.

COVID-19 respiratory failure. Nonclinical data suggesting that VIP upregulates surfactant, inhibits cytokine synthesis and protects pulmonary cells drove rapid clinical testing during the pandemic. Youssef and colleagues (Critical Care Medicine, 2022) randomized 196 patients with COVID-19 respiratory failure at ten US hospitals in a 2:1 ratio to three days of intravenous aviptadil or placebo. The primary endpoint, being alive and free from respiratory failure at day 60, did not reach statistical significance.

The larger and more rigorous test came from the ACTIV-3b/TESICO platform trial, published in The Lancet Respiratory Medicine in 2023. In the aviptadil comparison, 461 participants with COVID-19-associated hypoxemic respiratory failure received a daily 12-hour infusion of aviptadil or placebo for three days, with a six-category ordinal outcome at 90 days as the primary endpoint. There was no evidence of benefit on the primary or secondary endpoints. An accompanying commentary in the same journal described it plainly as a negative trial for vasoactive intestinal peptide in this setting.

Regulatory outcome. On November 4, 2021, the FDA declined to issue an emergency use authorization for the aviptadil product then branded ZYESAMI in patients with critical COVID-19 and respiratory failure, citing insufficient data on known and potential benefits and risks. Taken together, the human respiratory evidence for aviptadil is negative or inconclusive, not positive.

This matters for how VIP is marketed online. The existence of registered trials is frequently presented as if it were evidence of efficacy. In this case the trials were run, they were reasonably large, and they did not demonstrate the hoped-for effect.

There is one genuine approved human use of VIP, and it is narrow. A fixed combination of aviptadil and phentolamine mesilate, marketed as Invicorp, is authorized for intracavernosal injection in the treatment of erectile dysfunction. Approval is national rather than FDA-based: the product has been licensed in the United Kingdom and in several Nordic and European markets, and is not an FDA-approved product in the United States.

The pharmacological logic is complementary. Phentolamine, an alpha-adrenergic antagonist, increases arterial inflow, while the VIP component contributes to smooth muscle relaxation and reduced venous outflow. VIP-containing nerve fibres are abundant in cavernosal tissue, so this is one of the few settings where VIP is delivered exactly where its receptors are concentrated, bypassing the half-life problem entirely.

The pivotal evidence comes from a double-blind, placebo-controlled study of intracavernosal VIP plus phentolamine delivered by auto-injector in men with non-psychogenic erectile dysfunction, published in the International Journal of Impotence Research in 1999 by Dinsmore and colleagues. Across 304 patients who completed a dose-assessment phase, the two fixed combinations tested produced response rates of 75.1% and 66.5% against 12% and 10.3% for placebo respectively, both statistically significant, with a median erection duration of 54 minutes. The principal adverse event was transient facial flushing, reported in 33.9% of injections, and priapism was rare.

Readers comparing peptide approaches to sexual function may also want to look at our separate monograph on PT-141 (bremelanotide), which works through an entirely different mechanism, acting centrally on melanocortin receptors rather than locally on vascular smooth muscle.

Approved status here applies to a specific prescription combination product, administered by a specific route, under medical supervision. It does not extend to research-grade VIP powder, to nasal sprays, or to any other preparation.

Much of the current consumer interest in VIP comes from its place in the CIRS (chronic inflammatory response syndrome) protocol associated with Dr. Ritchie Shoemaker. In that framework, chronic multi-system symptoms following exposure to water-damaged buildings and biotoxins are attributed to persistent innate immune activation, tracked using laboratory markers such as C4a, TGF-beta 1, VEGF and MMP-9. Low measured VIP is described as a common finding in these patients, and intranasal VIP is positioned as the final step, used after the earlier steps of the protocol have been completed.

The human data supporting this step is limited. The core publication reported an open-label case series of 20 patients treated with intranasal VIP for an extended period, with the authors describing reductions in refractory symptoms, improvements in the inflammatory markers listed above, improvement in exercise pulmonary artery pressure, better quality of life, and good tolerability. It appeared in 2013 in a journal that is not indexed in PubMed/MEDLINE.

Several limitations deserve to be stated explicitly rather than buried. There was no control group and no blinding, so placebo effects, regression to the mean and concurrent protocol steps cannot be separated from any drug effect. The sample was very small. The primary outcomes were biomarkers rather than validated clinical endpoints, and the biomarkers themselves are not generally accepted as measures of disease activity outside this framework. CIRS as a diagnostic entity is itself not recognized in mainstream diagnostic classifications, and no independent research group has replicated the trial.

Where VIP is used this way, it is as an unapproved, off-label, compounded preparation. That raises practical issues of pharmacy quality, sterility, peptide stability in nasal formulation, and the absence of any regulator-reviewed safety dataset for chronic intranasal administration. The honest summary is that the biological rationale is coherent and the clinical evidence is weak. A single uncontrolled series of 20 people is a hypothesis, not a demonstrated treatment.

We present this because readers encounter it and deserve an accurate appraisal, not because it constitutes a validated use. Decisions about suspected biotoxin illness belong with a qualified physician who can also exclude the many conventional diagnoses that produce overlapping symptoms.

The adverse effect profile of VIP follows directly from its pharmacology. Because it is a potent vasodilator, systemic administration is associated with flushing, hypotension, reflex tachycardia and headache. In the intracavernosal combination studied for erectile dysfunction, transient facial flushing was the dominant adverse event, reported in roughly a third of injections, and the incidence of priapism was very low. Local reactions and injection-site effects apply to any injected peptide.

Because VIP stimulates intestinal water and electrolyte secretion, gastrointestinal effects are predictable. The clearest illustration is pathological rather than pharmacological. VIP-secreting neuroendocrine tumours, or VIPomas, cause the watery diarrhea, hypokalemia and achlorhydria syndrome first described by Verner and Morrison. As reviewed by Grier in the Southern Medical Journal (1995), onset is insidious, diagnosis is often delayed by months to years, and morbidity arises from sustained dehydration with electrolyte and acid-base disturbance that can progress to chronic renal failure. That is what chronic VIP excess looks like, and it is a useful corrective to the idea that more VIP is simply better.

In the COVID-19 trials, intravenous aviptadil was given under intensive care monitoring precisely because of these hemodynamic and gastrointestinal effects. Hypotension and diarrhea are the effects clinicians watch for. People with low baseline blood pressure, significant cardiac disease, or conditions in which vasodilation is hazardous are exactly the people for whom this profile matters most.

A separate category of risk applies to material sold for research use. Research-grade peptides are not manufactured to pharmaceutical standards, and purity, correct sequence, sterility, endotoxin content and stability are not guaranteed. There is no prescriber, no pharmacovigilance and no recourse. Our article on whether peptides are safe goes through these quality and oversight questions in more detail.

This is educational content and not medical advice. Consult a healthcare professional before considering anything discussed here, and treat the absence of reported side effects in small studies as a limit of the data rather than evidence of safety.

VIP itself is an endogenous human peptide, so the regulatory question is always about a specific product and route. As a standalone medicine, VIP is not approved by the FDA or the EMA for any indication. Aviptadil remains an investigational drug in the United States, where the FDA declined an emergency use authorization in COVID-19 in November 2021 and has not since approved it for any use.

The combination of aviptadil with phentolamine for intracavernosal treatment of erectile dysfunction holds national marketing authorizations in a small number of European countries and is dispensed on prescription there. That approval is specific to that product, that indication and that route, and it does not confer legal status on VIP sold in any other form.

Everything else, including lyophilized VIP powder and compounded nasal sprays, sits outside approved medicine. Material sold online is typically labelled for laboratory research use only and is not intended for human administration. The legality of importing, possessing or administering such material varies by jurisdiction, and in many countries supplying an unapproved peptide for human use is a regulatory offence regardless of how the label is worded. Competitive athletes should verify the current status of any peptide against the prevailing anti-doping list before use, since lists are revised annually.

A final point on interpretation. VIP occupies an unusual position: the basic science is strong and decades deep, the physiological roles are genuinely established, and the clinical translation has mostly failed, largely because of pharmacokinetics. Those three statements are all true simultaneously. Marketing material tends to quote the first two and omit the third.

Medical disclaimer: this article is for educational purposes only and does not constitute medical advice, a dosing recommendation or a protocol. VIP is not an approved standalone therapy. Preclinical and animal findings do not establish human benefit. Consult a qualified healthcare professional before making any health decision, and do not self-administer unapproved substances.