- For in vitro testing and laboratory use only.
- Not for human or animal consumption.
- Bodily introduction is illegal.
- Handle only by licensed professionals.
- Not a drug, food, or cosmetic.
- Educational use only.
Quick take on Met-Enkephalin (MENK / MET5)
Met-Enkephalin — also called MENK, MET5, or Opioid Growth Factor (OGF) — is a naturally occurring pentapeptide discovered in 1975 by Scottish researchers John Hughes and Hans Kosterlitz, who were hunting for the body's endogenous opioid system. Your brain and adrenal glands make this peptide every day; synthetic versions are used in research and increasingly in clinical oncology settings, mostly in China. CAS 58569-55-4 is the free-base form.
Mechanism in plain English
MENK binds to two distinct systems. At delta-opioid receptors in the nervous system, it produces mild analgesic and mood-modulating effects. But the more interesting action is through OGFr (Opioid Growth Factor receptor), a separate receptor found on immune cells and tumor cells, where MENK does something counterintuitive for an opioid: it modulates immune function and suppresses tumor growth. Not through pain relief — through direct cellular signaling on T-cells, macrophages, and cancer cells.
What it's used for
People take it for two reasons:
- Immune modulation — MENK activates CD4+ and CD8+ T cells, suppresses regulatory T cells (Tregs) that dampen anti-tumor immunity, and shifts macrophages from pro-tumor (M2) to anti-tumor (M1) phenotypes.
- Cancer adjuvant therapy — dozens of clinical and preclinical studies, largely from Chinese research groups led by Fengping Shan's lab at China Medical University, show MENK inhibiting gastric, lung, colorectal, cervical, and pancreatic cancer growth, often synergizing with chemotherapy.
Upsides and downsides
Main upside — one of the cleanest safety profiles in this entire category. Endogenous peptide, non-toxic in clinical trials, non-addictive despite being an opioid (the OGFr pathway isn't the addictive one), and genuinely promising clinical data in immunocompromised patients and cancer adjuvant settings.
Main downside — very short half-life (minutes in circulation), requires frequent dosing, and the evidence base is concentrated in Chinese research rather than Western clinical trials. Classical "promising but under-replicated" peptide.
Typical protocol
Research protocols run 5-10 mg subcutaneously daily or every other day, often in 2-4 week cycles. Some cancer adjuvant protocols use intravenous infusion at higher doses under medical supervision.
Who should skip it
- Anyone on opioid-antagonist medications like naltrexone — which blocks MENK's anti-tumor effects by design.
- Anyone with active opioid addiction recovery concerns.
- Pregnant women.
Regulatory status
Not on WADA's prohibited list — no performance-enhancing effects. Not approved as a medication in the West; used clinically in China and in experimental oncology settings.
In 1975, a British neuroscientist named John Hughes was trying to find the body's endogenous morphine — the internal molecule that opiate drugs were somehow mimicking. Nobody knew if such a thing existed. Hughes ground up pig brains, purified the extracts, and in a landmark Nature paper announced he'd found it: two tiny pentapeptides that bound opioid receptors and produced analgesic effects. He called them enkephalins — from the Greek "in the head."
The two compounds differed by just one amino acid. One ended in leucine (leucine-enkephalin). The other ended in methionine. That second molecule — [Met5]-enkephalin, or MET5 for short — turned out to have a second career that nobody anticipated in 1975. Beyond pain relief, it also does something much stranger: it puts the brakes on cell division.
For four decades, researchers — most notably Ian Zagon and Patricia McLaughlin at Penn State — have characterized MET5 as an endogenous tumor suppressor. Under the name Opioid Growth Factor (OGF), it's gone through Phase I and Phase II clinical trials for pancreatic cancer. This isn't a gym peptide. It's a genuinely different kind of tool, sitting at the intersection of neuroendocrinology and oncology.
MET5: what it is and how it works in a nutshell
MET5 is an endogenous pentapeptide (sequence: Tyr-Gly-Gly-Phe-Met) — just five amino acids, naturally produced in your body from the pre-enkephalin precursor protein. Depending on what research community you ask, it goes by several names:
- [Met5]-enkephalin or Met-enkephalin — classical pharmacology name
- MENK — common abbreviation in modern literature
- Opioid Growth Factor (OGF) — the name used when discussing its cell-proliferation-inhibiting effects
- MET5 — shorthand used in peptide research
Same molecule, different names for different contexts. When you see OGF in cancer research papers, it's the same thing as MENK in immunology papers and [Met5]-enkephalin in pharmacology papers.
MET5 is not approved as a standalone therapeutic anywhere, but it's been investigated in Phase I and Phase II clinical trials for pancreatic cancer, and its metabolite-based applications continue to be explored. It sits in an unusual regulatory position — a genuinely clinically-studied compound that hasn't crossed the finish line, but isn't purely research-chemical either.
MET5 mechanism of action: what it actually does in the body
MET5 operates through two distinct biological systems — and this dual action is what makes it interesting:
1. Cell proliferation regulation via the OGF-OGFr axis
This is MET5's most unique function. It binds to the opioid growth factor receptor (OGFr) — originally called the zeta (ζ) opioid receptor — which is structurally and functionally separate from the classical mu, delta, and kappa opioid receptors [1]. When MET5 binds OGFr, the complex enters the nucleus and delays the G1/S transition of the cell cycle, effectively slowing or stopping cell division.
This matters enormously in cancer biology. Cancer cells are defined by uncontrolled proliferation. Zagon and McLaughlin's work has shown that:
- Exogenous MET5 (OGF) represses growth of human pancreatic cancer cells in culture and in nude mouse models
- Combining OGF with chemotherapy (gemcitabine, 5-fluorouracil) enhances DNA synthesis inhibition and tumor growth suppression
- The effect is specific to OGFr — molecular knockout of the receptor eliminates the response [2]
The 2010 Phase II trial by Smith et al. in advanced pancreatic cancer patients who had failed standard chemotherapy: 24 subjects received weekly OGF 250 µg/kg IV. Some measurable clinical benefit was observed, though this is exactly the kind of study where effect sizes are modest and populations are terminally ill [3].
2. Immune modulation via classical opioid receptors on immune cells
MET5 also binds mu, delta, and kappa opioid receptors on immune cells including T-cells, NK cells, macrophages, and dendritic cells [4]. This produces measurable immune effects:
- Activates natural killer (NK) cells and CD8+ cytotoxic T cells — the cells that actually kill tumor cells and virus-infected cells
- Inhibits regulatory T cells (Tregs) by suppressing Foxp3 expression and Smad2/3 phosphorylation — reducing immune suppression that tumors use to hide
- Shifts macrophage polarization from tumor-promoting M2 toward tumor-fighting M1 phenotype
- Reshapes the tumor microenvironment toward an anti-tumor immune state [5]
The antiviral effects are also documented — MET5 has shown activity against Herpes, HIV, CMV, coronavirus, influenza A, and Japanese encephalitis in various preclinical models [6].
What it doesn't do that you'd expect an opioid to do: MET5 has a very short half-life in circulation (minutes), which limits classical opioid effects like significant analgesia or euphoria at therapeutic doses. The anti-proliferative and immune effects happen at doses below those producing meaningful analgesia. It's an opioid peptide, but not primarily a "feel good" opioid.
Who uses MET5 and what for
MET5 sits in a narrow, specialized niche — nothing like the broad off-label use of GH peptides or Ipamorelin. Realistic user categories:
- Cancer research and clinical trial contexts — the primary legitimate use. Pancreatic cancer trials have been the main focus, with ongoing interest in lung, colorectal, gastric, and cervical cancer applications.
- Integrative oncology practitioners — some physicians working in complementary cancer care use MET5 or related compounds (low-dose naltrexone, which works through similar pathways) as adjunct therapy.
- Researchers studying the neuro-immune axis — MET5 is a standard tool for probing how endogenous opioid peptides influence immune function.
- Occasional biohacker use for general immune support — uncommon, poorly established, and not supported by data for healthy adults.
Realistic expectations are highly context-dependent. In cancer research settings with appropriate protocols, measurable effects on tumor markers and immune cell populations have been documented. In general wellness use, the evidence is essentially absent.
What WON'T happen: dramatic analgesic effects (doses used don't produce significant pain relief), general "immune boost" that makes you feel different day-to-day, any effect comparable to what people expect from GH or performance peptides. MET5 is a specialized research compound with specialized uses.
Honest framing: this peptide's legitimate evidence base is narrow but real. It has more published clinical trial data than most peptides in the "research chemical" market, but the data is specifically about cancer contexts, not general wellness.
What MET5 stacks with: researched combinations
The combinations that appear in published research are almost entirely oncology-focused:
- MET5 + Chemotherapy (gemcitabine, 5-FU) — the combination with the most published data. Additive effect on tumor growth inhibition in pancreatic cancer models.
- MET5 + Low-dose naltrexone (LDN) — based on the principle that brief opioid receptor blockade upregulates OGFr expression, potentiating MET5's effect. A concept explored by Zagon's group.
- MET5 + Standard immunotherapy agents — theoretical combinations with checkpoint inhibitors, given MET5's effect on PD-1/PD-L1 expression, but clinical data is limited.
Outside oncology, there are no well-established stacking protocols for MET5.
MET5 side effects and risks
Clinical trial data reports a remarkably benign safety profile compared to standard chemotherapy or most other opioid-related compounds.
What's been documented in human trials:
- Injection site reactions — common, usually mild
- Mild fatigue — occasionally reported
- Transient hypotension — especially with IV infusion at higher doses
- Mild nausea — less common than with most chemotherapies
What's NOT prominently reported — and this is actually notable — MET5 doesn't produce the classical opioid side effects (significant respiratory depression, dependence, euphoria, constipation) at therapeutic doses, likely because of its very short circulating half-life.
The paradoxical tumor concern. Here's the nuance the peptide space often skips: MET5's role in tumor responses is genuinely paradoxical in some contexts. A 2020 review in European Journal of Pharmacology specifically titled "The paradoxical role of methionine enkephalin in tumor responses" documents both anti-tumor effects (enhanced immune response, direct anti-proliferative action) and pro-tumor effects (inhibiting T and B cell proliferation in some contexts, potentially promoting tumor cell growth by binding opioid receptors on tumor cells, inducing immune tolerance) [7]. The outcome depends heavily on tumor type, dose, timing, and receptor expression.
This is a peptide where "more is better" is definitely wrong — dosing matters, and specific clinical contexts matter.
Who should be cautious or avoid:
- Anyone currently on opioid medications (interactions unpredictable)
- Pregnant or breastfeeding women (no safety data)
- People with known opioid sensitivity or opioid receptor abnormalities
- Anyone with a history of hormone-sensitive cancers without oncology supervision
- Non-cancer patients using MET5 speculatively — there's essentially no risk-benefit data supporting general wellness use
How to use and store MET5
Administration in published research is primarily intravenous infusion in clinical trial contexts, with subcutaneous injection emerging in off-label settings.
Protocols from clinical research:
- Published Phase II cancer protocol: OGF 250 μg/kg IV once weekly (Smith et al., 2010)
- Subcutaneous off-label use: 5-10 mg daily, though protocols vary widely and aren't standardized
- Cycle: in clinical trial contexts, treatment continues as long as tolerated and showing benefit. Short cycles aren't typically used.
- Timing: morning dosing common, though the short half-life means timing isn't critical
Storage: lyophilized powder in freezer at -20°C. After reconstitution with bacteriostatic water, refrigerate at 2-8°C and use within 14-21 days.
An important practical caveat: MET5 has a very short half-life in blood (minutes), which is part of why clinical trials used IV infusion rather than subcutaneous dosing. At-home SC use almost certainly produces different pharmacokinetics than the published IV protocols.
MET5 vs alternatives: what's different
- Low-dose naltrexone (LDN) — an opioid receptor antagonist that produces brief blockade followed by upregulation of endogenous opioid activity including OGF/OGFr. Oral, cheap, prescription-available in many countries. Different mechanism, similar downstream target. Much more established clinical data in autoimmune disease and some cancer contexts.
- Standard chemotherapy — direct cytotoxic effects, far more potent for tumor reduction, but with severe side effects. Different tool for different stages of cancer treatment.
- Immune checkpoint inhibitors (anti-PD-1, anti-CTLA-4) — modern immunotherapy with dramatic effects in some cancers. Very different mechanism, much more potent, much more expensive and side-effect-heavy.
- Thymosin alpha-1 — immune-modulating peptide with different mechanism. Used adjunctively in some cancer contexts.
MET5's distinguishing feature: an endogenous peptide that simultaneously addresses tumor proliferation directly and modulates anti-tumor immunity. This dual action is unique in the peptide world, even if the effect size is modest compared to modern cancer treatments.
Myths about MET5
- "MET5 is a natural opioid so it's a safer painkiller." It's not used for pain in any practical protocol. The short half-life and doses used in research contexts don't produce meaningful analgesia. Anyone thinking MET5 might be a "natural alternative" to opioid analgesics is confused about how the compound actually works.
- "MET5 cures cancer." The evidence supports modest anti-tumor effects in specific contexts — not cure. The Phase II pancreatic cancer data showed some clinical benefit in patients who had already failed standard therapy; it didn't produce tumor elimination. Calling MET5 a cancer cure misrepresents both the science and the population in whom it's been studied.
Sources
- Zagon, I. S., Verderame, M. F., & McLaughlin, P. J. (2002). The biology of the opioid growth factor receptor (OGFr). Brain Research Reviews, 38(3), 351-376. — foundational characterization of OGFr as the mediator of MET5's anti-proliferative effects.
- Zagon, I. S., Donahue, R. N., & McLaughlin, P. J. (2009). Opioid growth factor-opioid growth factor receptor axis is a physiological determinant of cell proliferation in diverse human cancers. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, 297(4), R1154-R1161. — establishes the OGF-OGFr axis as a general regulator of tumor cell proliferation.
- Smith, J. P., Conter, R. L., Bingaman, S. I., Harvey, H. A., Mauger, D. T., Ahmad, M., Demers, L. M., Stanley, W. B., McLaughlin, P. J., & Zagon, I. S. (2010). Treatment of advanced pancreatic cancer with opioid growth factor: phase I. Anti-Cancer Drugs, 15(3), 203-209. — key clinical trial data on OGF in advanced pancreatic cancer.
- Zhao, D., Plotnikoff, N., Griffin, N., Song, T., & Shan, F. (2016). Methionine enkephalin, its role in immunoregulation and cancer therapy. International Immunopharmacology, 37, 59-64. https://pubmed.ncbi.nlm.nih.gov/26927200/ — comprehensive review of MENK/MET5 immunoregulatory effects.
- Zhang, S., Huang, H., Handley, M., Griffin, N., Bai, X., & Shan, F. (2021). A novel mechanism of lung cancer inhibition by methionine enkephalin through remodeling the immune status of the tumor microenvironment. International Immunopharmacology, 99, 107999. https://pubmed.ncbi.nlm.nih.gov/34315116/ — documents tumor microenvironment remodeling effects.
- Plotnikoff, N. P., Faith, R. E., Murgo, A. F., Herberman, R. B., & Good, R. A. (1997). Methionine enkephalin: a new cytokine — human studies. Clinical Immunology and Immunopathology, 82(2), 93-101. — foundational human studies documenting immune effects and antiviral activity.
- Tuo, Y., Tian, C., Lu, L., & Xiang, M. (2020). The paradoxical role of methionine enkephalin in tumor responses. European Journal of Pharmacology, 882, 173253. https://pubmed.ncbi.nlm.nih.gov/32535097/ — important review documenting the complex, sometimes contradictory, effects of MENK in tumor biology.
- Hughes, J., Smith, T. W., Kosterlitz, H. W., Fothergill, L. A., Morgan, B. A., & Morris, H. R. (1975). Identification of two related pentapeptides from the brain with potent opiate agonist activity. Nature, 258(5536), 577-580. — original discovery paper identifying met-enkephalin and leu-enkephalin.
Met-Enkephalin (MENK) Dosage Guide
Met-Enkephalin (MENK, also called Opioid Growth Factor or OGF) is an endogenous pentapeptide (Tyr-Gly-Gly-Phe-Met) that binds the delta-opioid receptor and the opioid growth factor receptor (OGFr), producing immunomodulatory, antinociceptive, and anti-tumor effects through T-cell activation, NK cell enhancement, and Treg suppression. This guide is aimed at researchers exploring MENK for immune support during chemotherapy, users studying its adjunct role in cancer immunotherapy, and those investigating pain modulation or metabolic applications. Dosing below combines the Phase I/II clinical trials in advanced pancreatic cancer (Zagon and McLaughlin group), the 50-patient Chinese clinical study on lymphocyte subpopulations (Shan et al.), and the preclinical 20 mg/kg IP protocols from colorectal and lung cancer models.
Real-World Dosage Protocols by Experience Level
| Experience Level | Dose | Frequency | Notes |
|---|---|---|---|
| Beginner research | 1–2 mg | Once daily, SC | First 1–2 weeks; assess tolerance |
| Standard research | 3–5 mg | Once daily, SC | Most common community protocol for immune support |
| Immune support / post-chemo | 5 mg | Once daily, SC | Clinical study range for cancer patients |
| Clinical IV infusion | 250 mcg/kg | IV daily or weekly | Phase I/II pancreatic cancer trial (McLaughlin) |
| Preclinical reference | 20 mg/kg | IP daily | Rodent colorectal/lung cancer studies; not for human scaling |
Doses also shift depending on the specific goal. The same peptide used for immune modulation versus pain management can follow quite different protocols.
Dosage by Goal
| Goal | Recommended Dose | Frequency | Cycle Length |
|---|---|---|---|
| Immune support (adjunct to chemotherapy) | 3–5 mg | Once daily, SC | 4–8 weeks with chemo cycles |
| Post-chemotherapy immune recovery | 3–5 mg | Once daily, SC | 4–6 weeks |
| Cancer immunotherapy research | 250 mcg/kg | IV infusion | Clinical setting only, 21-day cycles |
| Pain modulation | 1–3 mg | 1–2 times daily, SC | As needed, short cycles |
| Anti-viral / post-vaccine immune support | 2–3 mg | Once daily, SC or intranasal | 2–4 weeks |
| Metabolic support (adipocyte browning) | 2–3 mg | Once daily, SC | 4–8 weeks |
Be aware that MENK has an extremely short half-life (approximately 2–7 minutes in circulation), which is why clinical protocols use continuous IV infusion rather than bolus dosing for oncology applications — subcutaneous community protocols rely on local tissue effects and receptor-mediated signaling rather than sustained blood levels. Unlike most peptides on this list, MENK's anti-tumor mechanism depends on OGFr binding, and the dose-response curve is non-linear with effects documented at nanomolar concentrations in vitro. Absolute contraindications include concurrent opioid antagonist use (naloxone, naltrexone blocks the receptor), active opioid addiction or recent withdrawal, and patients with rapidly growing tumors where cancer type and OGFr expression status have not been characterized — consult an oncologist before use in any cancer context.
Met-Enkephalin Storage Guide: How to Keep Your Research Peptide Stable and Effective
Met-enkephalin ships as a white lyophilized powder in a sealed glass vial, freeze-dried to preserve its 5-amino-acid opioid peptide structure (Tyr-Gly-Gly-Phe-Met) and extend its shelf life. With a few simple habits — cold, dark, dry — the sealed vial stays in perfect condition for its full shelf life. Here's exactly how to store it.
Lyophilized Powder (Unreconstituted)
| Parameter | Details | Notes |
|---|---|---|
| Storage Temperature | Freezer at −20°C (−4°F) for long-term storage up to 24 months. Refrigeration at 2–8°C (36–46°F) is fine for short-term use up to ~3 months. | Original sealed vial in the freezer is the safest default. |
| Light Sensitivity | Yes — Met-enkephalin contains tyrosine and methionine residues that are prone to photodegradation. | Always keep in the original box or an opaque, amber container. |
| Freezing | Allowed and recommended. −20°C is standard for long-term storage; −80°C extends stability further if available. | Freeze from the start if you won't use it within 3 months. |
| Oxidation Sensitivity | The methionine residue is highly oxidation-prone (converting to methionine sulfoxide on air exposure), making a tight seal essential. | Keep the aluminum crimp cap intact until ready to reconstitute. |
| Signs of Degradation | Healthy powder is white to off-white and loose or cake-like. Watch for yellowing, browning, clumping, visible moisture, or a sticky texture. | Any color change, clumping, or moisture = discard the vial. |
| Common Mistakes | Leaving the vial at room temperature after delivery, storing in a humid kitchen or bathroom, or opening a cold vial and letting condensation form inside. | Put it in the freezer on arrival, and let sealed vials warm to room temperature before opening. |
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Shipping Times
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|---|---|---|
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Outer packaging is neutral and does not display product details on the exterior — a common approach to protect shipments from damage, tampering, and unnecessary exposure during delivery.
What to Expect
- Orders are processed after payment confirmation
- USA domestic shipping is typically faster when local stock is selected
- International orders include tracking, though update frequency may vary by destination
- Multiple warehouses may result in separate shipments when applicable
Authenticity & Verified Supply
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| Authenticity Feature | Details |
|---|---|
| Packaging | Original manufacturer packaging — sealed and unaltered |
| Lab Documentation | Batch-linked certificate of analysis available on request |
| Supply Chain | Sourced exclusively through official Generic Peptides distribution |
Shipping & Returns
MET5, more formally known as Met-enkephalin or Met5-enkephalin, is a naturally occurring opioid peptide made by your own body. It is a short chain of just five amino acids — Tyr-Gly-Gly-Phe-Met — and was discovered in 1975 by John Hughes and Hans Kosterlitz, who were searching for the body's natural "morphine-like" molecules. It belongs to the enkephalin family alongside its close cousin Leu-enkephalin. When studied in the context of cell growth rather than pain, the same peptide is often called Opioid Growth Factor (OGF).
MET5 acts as a potent agonist at the delta-opioid receptor (DOR), and to a lesser extent at the mu-opioid receptor (MOR), with little activity at the kappa receptor. When it binds to these G-protein coupled receptors, it inhibits adenylate cyclase, lowers cAMP inside nerve cells, and modulates ion channels — the end result is reduced neuronal excitability and decreased pain signal transmission. Separately, MET5 binds to the OGF receptor (OGFr), a completely distinct receptor located on the outer nuclear envelope, where it slows cell division by upregulating the p16 and p21 cyclin-dependent kinase inhibitors.
Research suggests MET5 has three main areas of potential benefit: analgesia (pain relief without classic opioid dependence), immune modulation, and anti-cancer activity. The most compelling clinical data comes from pancreatic cancer research — a Phase II trial (NCT00109941) reported that patients with advanced pancreatic cancer who received Met-enkephalin had a median survival time roughly three times longer than untreated controls. MET5 also appears to inhibit tumor angiogenesis (blood vessel growth that feeds cancers) and shows anti-inflammatory effects relevant to autoimmune conditions. Human data is still limited, however, so these findings are promising but not yet definitive.
Met-enkephalin is one of the body's built-in pain-control systems, and elevated enkephalin levels are associated with reduced chronic pain in conditions like fibromyalgia. In theory, delivering it as a drug could provide opioid-like analgesia with less risk of respiratory depression and addiction than morphine. In practice, this has been difficult because MET5 has an extremely short half-life (minutes) and is rapidly destroyed by enzymes called enkephalinases. Most current pain research focuses on stable analogs or enkephalinase inhibitors rather than the raw peptide itself.
Evidence in this area is genuinely interesting but still early. When acting as Opioid Growth Factor, MET5 binds to OGFr and slows the cell cycle at the G1/S checkpoint, which has been shown in both animal models and human trials to suppress growth of pancreatic cancer, melanoma, and several other tumor types. A landmark NIH-sponsored trial in advanced pancreatic cancer showed a meaningful survival benefit. That said, MET5 is not an approved cancer treatment, the studies are small, and it should never replace standard oncology care — it is at most a potential adjunct therapy under active research.
There is no officially approved dose for MET5. In the pancreatic cancer trials, patients received Met-enkephalin intravenously, typically at doses in the range of 250 mcg/kg. In research peptide protocols, subcutaneous injection is more common, with doses varying widely based on the research goal. Because the peptide has a half-life of only a few minutes, frequent dosing or continuous infusion is usually required to maintain blood levels — a practical limitation that has slowed its development as a finished drug.
In clinical trials, Met-enkephalin has been remarkably well tolerated, with side effects generally milder than those of traditional opioid painkillers. Reported adverse effects include mild dizziness, fatigue, transient drops in blood pressure during infusion, and occasional gastrointestinal changes such as slowed bowel motility. Notably, it does not appear to produce the euphoria, respiratory depression, or strong addiction potential associated with mu-opioid-selective drugs like morphine, because of its preference for the delta receptor. Long-term human safety data remains limited.
The addiction risk with MET5 is believed to be considerably lower than with mu-selective opioids such as morphine or oxycodone, because its main activity is at the delta-opioid receptor, which is less involved in reward and euphoria. However, "lower risk" is not "zero risk" — the enkephalinergic system is involved in the brain's reward pathway and plays a role in substance use disorders. Because human experience with MET5 as a long-term therapy is minimal, its true dependence potential in regular use is not fully characterized.
Both MET5 and endorphins are endogenous opioid peptides, but they differ in size, origin, and receptor preference. MET5 is a short 5-amino-acid peptide derived from the precursor protein proenkephalin and prefers delta-opioid receptors. Beta-endorphin is much larger (31 amino acids), derived from the precursor proopiomelanocortin (POMC), and acts mainly at mu-opioid receptors. You can think of endorphins as the body's "morphine equivalent" and enkephalins like MET5 as a shorter-acting, more receptor-selective family with broader roles in pain, mood, immunity, and cell growth.
Met-enkephalin is not an FDA-approved medication for any condition in most countries. It has been investigated in clinical trials — most notably under the name "MENK" or "OGF" in cancer research — but it remains an experimental compound. It is sold by research peptide suppliers labeled "not for human consumption," which exists in a legal grey zone in many jurisdictions. Because of its rapid degradation in the body and its experimental status, self-administration is neither safe nor practically useful in the way some users might hope.
