VIP

Vasoactive intestinal peptide (VIP) is a 28-amino-acid signaling peptide first isolated from pig intestine in 1970 and named for its ability to relax blood vessels (vasodilation) ¹. The body produces its own VIP in gut nerves, immune cells, and brain regions, where it coordinates intestinal secretion, blood vessel tone, the daily body clock, and the inflammatory response ¹.

Most clinical work has used the synthetic form, aviptadil, which reached a phase 2/3 trial in severe COVID-19 respiratory failure ³. The native peptide is also studied for inflammatory bowel disease, sepsis, acute lung injury, asthma and COPD, osteoarthritis, and neurodegenerative conditions ⁷.

VIP's effects are context dependent. It is broadly anti-inflammatory in most tissues, but it can also foster a pro-inflammatory T cell type (Th17) in certain settings, and some tumors appear to co-opt VIP signaling to hide from the immune system ¹¹. That duality runs through the evidence on this page.

VIP acts through two dedicated cell-surface receptors, VPAC1 and VPAC2. Both are G protein-coupled receptors: when VIP docks on, the receptor switches on an enzyme that raises cyclic AMP (cAMP), and cAMP relays instructions inside the cell ². A third related receptor, PAC1, is engaged only at concentrations above normal VIP levels ²⁵. The human VPAC1 receptor binds VIP with high affinity (about 2.5 nanomolar) ²⁶.

The anti-inflammatory signature

VIP's best-characterized action is against NF-κB, the master switch that turns on most inflammatory genes. In immune cells exposed to a bacterial trigger, VIP blocks NF-κB at three separate points: it stabilizes the inhibitor that holds NF-κB inactive in the cell fluid, it diverts a coactivator protein (CBP) away from NF-κB, and it disrupts the gene-reading machinery ²⁸. The result is less TNF-α, IL-6, IL-12, and nitric oxide synthase (iNOS), and more of the anti-inflammatory signal IL-10 ²⁹. VIP also quiets a second alarm complex, the NLRP3 inflammasome, reducing the mature forms of IL-1β and IL-18 ⁹. In mice, that adds up to protection from otherwise lethal septic shock ¹².

Beyond the immune system

VIP is one of the gut's main inhibitory nerves, relaxing intestinal muscle ³⁴. It also maintains the intestinal barrier: mice genetically missing VIP have fewer mucus-producing goblet cells, less of the protective proteins MUC2 and TFF3, and a leakier, more colitis-prone gut ⁸. In the brain's master clock, VIP is the signal that keeps daily rhythms synchronized ³⁵. And in blood vessels, the same cAMP wave relaxes the vessel wall, which is why flushing and lowered blood pressure are VIP's signature effects ⁴⁹.

Not simple immunosuppression

The immune picture cuts both ways. VIP can promote a pro-inflammatory T cell type, Th17, through VPAC1 ³². And in pancreatic cancer, tumors overproduce VIP; blocking, not stimulating, its receptors is what unleashes anti-tumor immunity in mice ¹¹.

VIP's documented effects sit in two evidence tiers: one registration-grade human trial in severe respiratory failure, and a broad preclinical literature.

The strongest human evidence

A 196-patient trial randomized critically ill COVID-19 patients 2:1 to 3 days of intravenous aviptadil or placebo ³. The primary endpoint, alive and free of respiratory failure at day 60, was not met (odds ratio 1.6; 95% CI 0.86–3.11). But 60-day survival roughly doubled (OR 2.0; 95% CI 1.1–3.9; p=0.035), the inflammatory marker IL-6 fell by day 3, and the ventilated subgroup showed a 10-fold survival odds increase (p=0.031). No drug-related serious adverse events were recorded ³. A 2025 systematic review and meta-analysis pooled aviptadil trials in acute respiratory distress syndrome to test whether the effect holds up ⁴.

Healthy-volunteer findings

A 2-hour intravenous infusion in 12 healthy volunteers dilated the scalp arteries for more than 2 hours, triggered tearing, and produced a delayed migraine-like headache that peaked 2–3 hours after the infusion and again at 4–12 hours ¹⁰. A dedicated episodic-migraine trial used the same model ⁵⁰.

Preclinical and observational evidence

• Gut barrier: mice without VIP have distorted intestinal crypts, depleted goblet cells, and increased permeability, and supplying VIP reverses the defect ⁸. A nanoengineered form (VIP-SSM) reversed colitis after a single dose where the free peptide failed ⁷.
• Host defense: VIP helps recruit immune cells that defend against the gut pathogen Citrobacter rodentium, while food-triggered VIP release tunes the same circuitry toward nutrient absorption ²².
• Sepsis and lung injury: VIP suppresses TNF-α and IL-6 and protects against lethal endotoxic shock in mice ¹²; it also quiets the NLRP3 inflammasome in acute lung injury ⁹.
• Joints: VIP is low in osteoarthritic joint fluid, and local VIP reduces pain signaling in rat osteoarthritis models ¹⁵.
• Brain: VIP blocks inflammatory microglial activation and protects neurons in a mouse Parkinson's model ¹⁷.
• Eye: intravitreal VIP slowed myopia progression in guinea pigs in a dose-dependent way ¹⁸.
• Metabolism: mice without VIP carry less body fat and more lean mass, and a human genetic analysis ranked the VIP pathway first for fat mass ¹⁴. VIP is not studied as an anabolic or body-composition agent ¹⁴.
• Cancer caution: pancreatic tumors overproduce VIP, and VIP receptor blockade helped immunotherapy eliminate tumors in up to 40% of mice ¹¹.

Diagnostic use

VIP-secreting tumors (VIPomas) present with relentless watery diarrhea, low potassium, and dehydration, which resolve after tumor removal ¹⁹. A high serum VIP level alone poorly predicts the diagnosis, and a serum-negative case is on record, so testing only makes sense in the right clinical picture ²¹.

Two human dosing patterns have been reported, and a community subcutaneous pattern exists without published data.

The registration-grade protocol. The pivotal trial treated critical COVID-19 respiratory failure with intravenous aviptadil for 3 consecutive days, following the intermittent-infusion schedule of protocol NCT04311697 ³. An expanded-access protocol (RLF-100-EA-1) covered patients who fell outside the main trial ⁴². This is an inpatient acute-care regimen; hypotension is the dose-limiting concern with free-peptide infusion ⁷.

The experimental infusion protocol. Twelve healthy volunteers received a single 2-hour continuous intravenous infusion to map the peptide's vascular and headache profile ¹⁰.

Community subcutaneous protocol. No published human data exist for subcutaneous use, so the practitioner-derived pattern is best read as convention, not literature: 50–100 mcg once or twice daily, run for 4–6 weeks, then 2–4 weeks off. Some practitioners start at about half of a weight-scaled reference of roughly 1 mcg/kg and titrate upward, with blood pressure as the limiting factor.

The logic behind short, intermittent courses is receptor desensitization. In a human melanoma cell line, VIP exposure pulled about 80% of surface binding sites inside the cell within minutes, with most recovery within an hour of washout ⁴⁸. In a mouse colitis model, alternate-day dosing was active ⁷.

Other routes appear in the record: inhaled aviptadil at roughly 50–100 mcg two to three times daily in community protocols, an inhaled COPD trial reporting improved 6-minute walk test and quality of life in a meeting abstract ⁴⁵, continued discussion of inhaled aviptadil for post-COVID lung injury ⁶⁹, intranasal delivery at 50–100 mcg per dose in community protocols, and ocular delivery in animal studies of retinoprotection and myopia ¹⁸. No validated weight-based human scheme exists; the trial used fixed-schedule dosing ³, and the only formal dose-response dataset is the guinea pig intraocular series (0.1 to 10,000 pmol, with 100 pmol optimal) ¹⁸.

What research monitoring watches for: blood pressure and flushing, delayed headache, and stool changes, given VIP's potency as an intestinal secretagogue ¹⁰.

VIP is supplied as a freeze-dried (lyophilized) powder; the clinical form, aviptadil, was prepared as an intravenous infusion in the pivotal trial ³. For research handling, bacteriostatic water (0.9% benzyl alcohol) is the standard diluent for multi-use vials, with sterile water an option that shortens the usable life of the mixed solution.

Preparation follows standard peptide practice. Let the lyophilized vial reach room temperature, drip the diluent down the wall of the vial, and swirl gently to dissolve. Do not vortex or shake, which can damage the peptide. For intravenous preparation, the clinical route uses dilution into sterile saline, with dosing approached cautiously because continuous infusion of the free peptide carries dose-limiting hypotension ⁷.

Storage rules are simple on both sides of mixing. Unreconstituted powder stores best frozen at −20°C for long-term storage, or at 2–8°C short-term, kept from light and humidity. A mixed solution should be refrigerated at 2–8°C and used within about 30 days in bacteriostatic water, or 7–14 days in sterile water. Avoid repeated freeze-thaw cycles and use glass vials where possible, since the peptide can stick to plastic. Discard any solution that turns cloudy.

Stability defines the handling problem. Free VIP loses activity within minutes in the body, and the engineering literature exists precisely to fix this: a cyclic disulfide analog held activity past 40 minutes where the open-chain form lasted about 5 minutes ³⁸, and micellar and stapled-peptide versions add stability and receptor selectivity ⁷. The calculator below handles the dilution arithmetic for typical vial sizes and doses.

No dedicated human drug-interaction studies exist for VIP outside the aviptadil clinical program, so the entries below carry their evidence level ³.

Drugs

• Blood pressure medications (moderate to high concern). VIP is a direct vasodilator, and hypotension limits continuous infusion ⁷. In newborn dogs, VIP plus the alpha-blocker phentolamine dropped blood pressure about 54%, versus 24% for phentolamine alone ⁶⁰. Practitioner guidance flags extra care with nitrates, PDE5 inhibitors, and alpha-blockers. The pivotal trial itself recorded no drug-related serious adverse events over 3 days of intravenous dosing under monitoring ³.
• Immune checkpoint inhibitors (theoretical but important). VIP signaling suppresses effector T cells, and blocking VIP receptors boosted anti-PD-1 tumor elimination in a pancreatic cancer mouse model ¹¹. Added VIP could plausibly blunt checkpoint therapy.
• Corticosteroids and immunosuppressants (moderate). VIP quiets NF-κB-driven inflammation, and glucocorticoids increase VIP receptor display on human immune cells, so effects may compound ²⁸. Dexamethasone also modulates VIP-evoked ACTH responses in Cushing's disease ⁶³.
• Insulin and glucose-lowering drugs (low to moderate, theoretical). VPAC2-selective VIP analogs increase glucose-dependent insulin secretion ³⁹.
• Somatostatin analogs. Somatostatin opposes VIP-stimulated cyclic AMP in human lymphocytes ⁶⁴, and octreotide is standard treatment for VIPoma diarrhea, making this combination a functional opposite rather than an interaction.
• Triptans (minimal). Sumatriptan did not change circulating VIP in healthy volunteers ⁵⁹, though VIP itself provokes headache ¹⁰.

Supplements

Purely theoretical, but consistent with mechanism: blood-pressure-lowering supplements (arginine, citrulline, beetroot nitrates), anti-inflammatory supplements (fish oil, curcumin, resveratrol), and glucose-lowering supplements (berberine, cinnamon) sit on the same axes VIP acts on. Alcohol adds vasodilation and gut secretion.

Conditions that matter

Hypotension or autonomic instability, migraine (reliably provoked in the infusion model ¹⁰), flushing disorders and MEN1-associated VIP-secreting tumors ⁴⁰, active cancer on checkpoint immunotherapy ¹¹, and prolactin-sensitive conditions, since VIP releases pituitary prolactin ⁵⁴.

Combination data are mostly mechanistic; no human trial has stacked VIP with other peptides.

• PACAP. VIP and PACAP are 68% similar and share the VPAC1 and VPAC2 receptors ¹. Adding PACAP adds the PAC1 receptor, which VIP only engages at high concentrations, plus the same vasodilatory burden, so the combination mainly compounds blood pressure effects ²⁵.
• GLP-2 (teduglutide). GLP-2 raises VIP expression in gut neurons through PI3K signaling, a plausible synergy for intestinal barrier and secretory repair ⁶⁵.
• GLP-1 drugs (semaglutide, tirzepatide). Both classes signal through cAMP in gut and metabolic tissues; mice without VIP show altered GLP-1, insulin, and satiety hormone profiles, so overlapping effects on transit and glucose are plausible ¹⁴.
• BPC-157 and TB-500. No direct data, but VIP maintains the colonic barrier in animal colitis models ⁸, and repair peptides are commonly co-studied in gut-healing research. No known conflict is documented.
• Thymosin alpha-1. It binds VIP receptors as a partial agonist in rat lymphocytes, so the two could compete rather than add ⁶⁶.
• GH secretagogues (ipamorelin, MK-677). VIP is a physiological prolactin-releasing factor ⁵⁴, so combining it with prolactinogenic peptides is a theoretical watch item.
• Melanotan II and PT-141. Additive vasodilation and flushing are the theoretical concern; no formal data exist.

The larger caution for stacking is VIP's bidirectional immune signature, anti-inflammatory in most settings but Th17 promoting in some ³². Combining immunomodulators multiplies unknowns.

No VIP formulation holds marketing approval in the US or Europe; the compound remains investigational ⁵. Its synthetic form, aviptadil (RLF-100), cleared FDA human-trial review in 2001 and the European agency in 2005, with Orphan Drug Designations for acute respiratory distress syndrome (2001) and pulmonary arterial hypertension (2005) ⁵. The FDA also granted Fast Track designation for critical COVID-19 respiratory failure, and an expanded-access protocol (RLF-100-EA-1) treated patients outside the pivotal trial ⁶.

The pivotal trial's mixed result, a survival benefit without the primary endpoint, means no marketing approval followed ³. A 2025 meta-analysis and editorial commentary weighed whether that result supports standard-of-care use; the conclusion remains equivocal ⁴.

VIP is not a controlled substance under the US Controlled Substances Act, and no country-specific ban appears in the reviewed record. Practical access runs through 503A/503B compounding pharmacies or gray-market research suppliers, marketed as not for human use. In sports, VIP is not on the WADA Prohibited List, and no athlete sanction involving VIP appears in the literature; there is also no documented ergogenic rationale, since its known effects are vascular, immune, and circadian ¹⁰.

The side-effect profile comes mainly from the 196-patient intravenous trial of aviptadil and a 12-volunteer infusion study ³.

Common and dose-related effects

• Delayed headache. A 2-hour infusion produced migraine-like headache that peaked 2–3 hours after the infusion and again at 4–12 hours, alongside sustained dilation of scalp arteries ¹⁰. A separate migraine trial used the same model ⁵⁰.
• Flushing and warmth. VIP is one of the principal mediators of flushing, via direct blood vessel relaxation ⁴¹.
• Low blood pressure and fast heart rate. This is the dose-limiting effect of free peptide in continuous infusion ⁷; the 3-day trial recorded no drug-related serious adverse events in critical-care patients ³.
• Loose stools. VIP drives intestinal secretion; sustained excess, as in VIP-secreting tumors, causes profuse watery diarrhea with low potassium ¹⁹.
• Sleepiness. VIP has REM sleep-promoting activity in animal studies ⁴⁷.

Less common or theoretical

• Tearing during infusion ¹⁰.
• Th17 skewing, since VIP can promote this pro-inflammatory T cell program through VPAC1 ³²; colitis models cut both ways ⁸.
• Cognitive effects are unresolved: rodent work shows memory impairment and cognitive dysfunction with VPAC2 overexpression, while a Parkinson's model shows neuroprotection ¹⁷.
• Metabolic signals lean toward fat storage in mice and humans ¹⁴.
• Glucose lowering and prolactin elevation are plausible from receptor pharmacology ³⁹.
• Cancer signals are mixed: transformation in a prostate cell line, inhibition in a renal cancer line, and a tumor-protective immune role in pancreatic cancer ¹¹.

Where research monitoring focuses

Blood pressure and heart rate around each dose, electrolytes if diarrhea develops, glucose in metabolic studies, and a headache and flushing log ³. Particular caution applies to hypotension or unstable cardiovascular status, migraine, hormone-sensitive cancers, inflammatory bowel disease, diabetes, and pregnancy, where no human data exist ¹⁰.