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Endomorphin-1 & Endomorphin-24 residuesYPWFEach bubble = one amino acid. Size = residue mass. Color = chemical class.Also known as: EM-1, EM-2, Endomorphin 1
Four amino acids. That's all it takes to build the most selective mu-opioid receptor agonist the human body produces. Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and Endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) bind MOR with 4,000 to 15,000 fold selectivity over delta and kappa receptors. Discovered by Zadina and colleagues in 1997 [1], these tetrapeptides still have no approved clinical application. The native forms degrade in plasma within minutes. One cyclized analog, CYT-1010, reached Phase I and showed analgesia without respiratory depression. Researchers studying opioid pharmacology and next-generation pain drug design track these peptides closely.
Endomorphin-1 (EM-1; Tyr-Pro-Trp-Phe-NH2) and Endomorphin-2 (EM-2; Tyr-Pro-Phe-Phe-NH2) are endogenous tetrapeptides and the most selective mu-opioid receptor agonists ever identified. Zadina's group isolated them from bovine and human brain tissue in 1997 [1]. Before that discovery, no one had found the endogenous ligand for the mu receptor, the same receptor morphine targets. EM-1 binds MOR at a Ki of roughly 360 pM. That's 4,000 fold selective over delta and 15,000 fold over kappa. EM-2 binds at about 690 pM, with even higher mu/delta selectivity at 13,400 fold. They differ by a single amino acid at position three (tryptophan vs. phenylalanine), yet that swap changes everything about their downstream signaling and distribution in the brain. In practice, these peptides are research tools. EM-1 concentrates in the nucleus accumbens, cortex, and dorsal root ganglia; EM-2 clusters in the spinal cord and lower brainstem. Both produce potent analgesia in rodent pain models when administered directly into the brain or spinal cord. Peripheral injection is mostly pointless; plasma dipeptidyl peptidase IV (DPP-IV) shreds the Tyr-Pro bond within 2 to 8 minutes. The real excitement is in the analogs. CYT-1010, a cyclized EM-1 derivative, completed Phase I and showed real analgesia in healthy volunteers without observed respiratory depression (Toll et al.)[2]. That is the holy grail of opioid drug design. PubMed lists 913 papers on endomorphins. No consumer peptide vendor sells them, and no self-dosing community exists. This is pure pharmacology research, and the data is genuinely promising for future pain therapeutics.
EM-1 and EM-2 both activate the mu-opioid receptor (MOR/OPRM1), but they do it differently. That difference matters. Both peptides trigger Gi/Go protein-coupled signaling. Adenylyl cyclase goes quiet. cAMP drops. Voltage-gated N-type and P/Q-type calcium channels close, reducing presynaptic neurotransmitter release. G protein-coupled inwardly rectifying potassium channels (GIRKs) open, hyperpolarizing the neuron. The net result is suppressed nociceptive signaling. EM-2 is a biased agonist. It preferentially recruits beta-arrestin over G protein signaling compared to reference agonists like DAMGO. Lower operational efficacy for G protein-mediated responses in mature neurons may explain the reduced respiratory depression and tolerance profiles spotted in preclinical models [3]. At the systems level, the two peptides split territory. EM-1 handles supraspinal analgesia through MOR in the periaqueductal gray, rostral ventromedial medulla, and thalamus. EM-2 engages descending pathways that release dynorphin A (kappa receptors) and met-enkephalin (delta-2 receptors), creating a multi-receptor analgesic cascade. EM-2 immunoreactive fibers sit right next to serotonergic neurons in the RVM, linking endomorphin signaling to descending serotonergic pain modulation. Outside pain, EM-2 produces hypotension and bradycardia through MOR activation in the nucleus tractus solitarius [4]. Both peptides suppress pro-inflammatory cytokine release via MOR on macrophages and lymphocytes, inhibiting p38 MAPK signaling [5].
Endomorphin-1 and Endomorphin-2 are the most selective endogenous mu-opioid receptor agonists identified. Potent antinociception in preclinical models is well-established. No validated human dosing for native peptides exists: all site dose tiers are speculative extrapolations. CYT-1010 (cyclized EM-1 analog) showed analgesia without respiratory depression in Phase I.
Zadina et al. 1997 (PMID 9087409): discovery of EM-1/EM-2 as MOR endogenous agonists; CYT-1010 Phase I (Toll et al.): first human data for an EM-1 analog showing analgesia without respiratory depression
No human PK/PD data for native endomorphins. Rapid plasma DPP-IV degradation (t½ ~2–8 min) renders peripheral dosing essentially futile without analog modification. No precursor gene identified: biosynthetic origin remains unresolved. Preclinical to human translation is uncertain for all opioid peptides.
No consumer or self-administration use documented. Endomorphin is not sold by research peptide vendors and has no self-dosing reports on Reddit, forums, or any grey-market channel. Essentially absent from community discourse.
All data is from published preclinical research and one Phase I study of an analog (CYT-1010). Zero consumer/community use. Community verdict fields reflect absence of data, not negative experience.
| Level | Dose / Injection | Frequency |
|---|---|---|
| Beginner | 50mcg | Single dose |
| Moderate | 100mcg | Single dose |
| Aggressive | 250mcg | Single dose |
This is a research reagent, not a consumer peptide. No standard vial exists at peptide vendors. If you're sourcing from a scientific supplier (GenScript, Bachem, AAPPTec), typical custom synthesis delivers 1 mg lyophilized. Add 2 mL bacteriostatic water for 500 mcg/mL. At that concentration, a 50 mcg dose pulls 10 units on a standard insulin syringe; 100 mcg pulls 20 units; 250 mcg pulls 50 units. Those doses are entirely speculative. No published human pharmacokinetic data supports any peripheral SC dose for native endomorphin. The Tyr-Pro bond gets cleaved by DPP-IV in minutes, and barely anything crosses the blood-brain barrier from a peripheral route. Swirl gently after reconstitution. Don't vortex. C-terminal amidation is what keeps these peptides binding MOR; rough handling won't help stability. Store lyophilized at -20C desiccated. Reconstituted, you get maybe 7 days at 2-8C. Prepare small aliquots and minimize freeze-thaw cycles. Request a CoA confirming sequence (EM-1: Tyr-Pro-Trp-Phe-NH2 or EM-2: Tyr-Pro-Phe-Phe-NH2) with HPLC purity at least 95% and mass spec verification.
No established human cycling protocol exists. The extremely short half-life of native endomorphins (minutes) makes sustained dosing impractical without stabilized analog formulations or continuous infusion. In preclinical repeated-dosing studies, tolerance develops within days. Any investigational protocol must include naloxone availability and medical supervision. Cycling parameters shown are theoretical placeholders based on opioid peptide class considerations.
Both EM-1 and EM-2 promote rapid MOR internalization via beta-arrestin-2 recruitment. EM-2 is a biased agonist with preferential beta-arrestin recruitment over G-protein signaling, making acute desensitization particularly rapid. Repeated dosing leads to progressive analgesic tolerance within 3–7 days in preclinical models. The 1-on/4-off placeholder in peptides.ts is entirely theoretical: no human cycling protocol or receptor recovery timeline has been established for native endomorphins. Standard opioid pharmacology suggests longer off-periods of 4–8 weeks for meaningful receptor re-sensitization, but this is extrapolated from morphine/opioid class data, not endomorphin-specific research.
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Expected: No validated human outcome data for native peptides. Preclinical: ~30 min analgesic window via central (ICV/IT) administration; very limited peripheral analgesic effect. CYT-1010 (analog) Phase I: significant analgesia vs. baseline without observed respiratory depression.
Monitor: Continuous respiratory rate and SpO2 monitoring required. Monitor BP and HR (EM-2 produces hypotension and bradycardia). Naloxone immediately accessible. Do not administer without medical supervision and emergency reversal capability.
Source lyophilized endomorphin from a scientific-grade peptide synthesis supplier. Confirm sequence and purity on the Certificate of Analysis before proceeding.
Reconstitute by adding 2 mL bacteriostatic water to a 1 mg vial. Swirl gently until dissolved. This gives 500 mcg/mL.
At 500 mcg/mL: 10 units equals 50 mcg (beginner tier), 20 units equals 100 mcg (moderate tier), 50 units equals 250 mcg (aggressive tier).
For a 2 mg vial reconstituted in 2 mL, the concentration doubles to 1,000 mcg/mL. Halve the syringe units: 5 units for 50 mcg, 10 units for 100 mcg, 25 units for 250 mcg.
Clean the site with an alcohol swab. Pinch the skin, insert at 45 degrees, inject slowly.
These dose tiers are speculative extrapolations from preclinical intracerebroventricular rodent data. No published human pharmacokinetic study validates SC dosing for native endomorphin.
Do not re-dose within 2 hours.
Have naloxone 0.4 mg IV/SC pre-positioned before any administration. Monitor respiratory rate and SpO2 continuously. Monitor blood pressure and heart rate at 5, 15, and 30 minutes post-dose.
Use within 7 days.
Orders of magnitude higher dose required vs. ICV to achieve any CNS analgesic effect; exact ratio not established in humans
DPP-IV in plasma degrades the Tyr-Pro bond within 2–8 minutes. BBB penetration is very low. SC administration is the most practical peripheral route but produces minimal central analgesia with native peptide. Stabilized analogs (D-amino acid, lipid-conjugated) are specifically designed to make SC viable.
Faster plasma exposure but same rapid DPP-IV degradation; any peripheral analgesic effect is transient (<5 min)
IV route is used in research settings for pharmacokinetic studies and cardiovascular effect measurements. Useful for studying peripheral opioid receptor effects on immune cells and gut. Does not bypass BBB.
Preclinical rodent: 0.1–30 nmol IT vs. 1–30 nmol ICV; spinal route engages EM-2 descending pathways more strongly
Requires sterile catheter placement and medical/surgical infrastructure. CSF degradation is slower than plasma (>2 hours for complete inactivation). Produces primarily spinal antinociception. EM-2 particularly active at spinal level. Only viable in medical research settings.
Rodent reference doses: 1–30 nmol ICV produce dose-dependent antinociception; EM-1 ~3× more potent than EM-2 supraspinally
Requires neurosurgical implantation of a cannula. Completely impractical outside laboratory animal research. Produces the clearest pharmacological data and is the source of most published endomorphin analgesic characterization. Not applicable to human self-administration.
Additive CNS and respiratory depression. Combined opioid + benzodiazepine is the leading cause of opioid overdose death. Potentially fatal.
Do not combineAdditive respiratory depression and overdose risk. Mu-opioid receptor saturation: no additional analgesic benefit, compounded toxicity.
Do not combineAdditive respiratory depression risk when combined with opioids: FDA black box warning class.
Do not combineRisk of unpredictable potentiation of opioid effects; can cause serotonin syndrome or severe respiratory/cardiovascular events.
Do not combineRespiratory depression is the leading cause of opioid overdose death, and endomorphins are mu-opioid agonists. Start there. Preclinical data does show reduced respiratory depression compared to morphine and DAMGO at equianalgesic doses. CYT-1010 (an EM-1 analog) reached Phase I with no observed respiratory depression. That is encouraging. It is not the same as safe. "Reduced" still means present, dose-dependent, and life-threatening if monitoring fails. Naloxone must be immediately accessible in any research setting; this is non-negotiable. Human data on native endomorphins is essentially absent. Total human exposure through published research is extremely small, concentrated in the CYT-1010 Phase I trial of the cyclized analog. Side effect frequency numbers from preclinical rodent work cannot be translated to human incidence rates with confidence. In animal models, observed effects include sedation, reduced locomotor activity, and motor impairment at higher doses. Gastrointestinal motility drops, producing constipation, though stabilized analogs show less GI impact than morphine. EM-2 produces hypotension through nucleus tractus solitarius activation and bradycardia through central autonomic pathways [4]. Blood pressure and heart rate monitoring is mandatory during any research administration. Tolerance to the analgesic effect develops with repeated dosing. Onset appears slower than morphine tolerance in head-to-head preclinical comparisons, but it still develops within 3 to 7 days. Physical dependence follows chronic exposure. Naloxone-precipitated withdrawal symptoms occur in dependent animals. EM-2 shows faster acute desensitization than EM-1 due to preferential beta-arrestin-2 recruitment and rapid MOR internalization. Re-dosing before receptor re-sensitization produces diminishing returns quickly. Urinary retention and miosis are expected opioid class effects. The extremely short plasma half-life (2 to 8 minutes for native forms) limits the duration of adverse effects after a single dose, which is a double-edged feature; it also limits any therapeutic benefit. When to stop and seek emergency care: respiratory rate below 10 breaths per minute, SpO2 below 92%, heart rate below 50 bpm, systolic blood pressure below 90 mmHg, excessive sedation, or loss of consciousness. Administer naloxone 0.4 mg IV or SC immediately. Pregnancy and breastfeeding: opioid peptides may cross the placental barrier. No safety data exists for any population, including pediatric.
Verify Endomorphin-1 & Endomorphin-2 dosing and safety with a second opinion
Not available from any consumer research peptide vendor. Only obtainable via custom synthesis from scientific reagent suppliers. No commercial vials exist for standard reconstitution. Purity and sterility entirely dependent on the synthesis lab. High risk of contamination or incorrect sequence in unvalidated sources. Storing a short-half-life tetrapeptide in reconstituted form risks rapid degradation even at -20°C if handling is suboptimal.
| Test | When | Target |
|---|---|---|
| Respiratory rate | Continuous during any research administration session | >12 breaths/min; alert at <10; treat at <8 |
| Pulse oximetry (SpO2) | Continuous during and 30 minutes post-administration | >95%; treat SpO2 <92% with naloxone and supplemental O2 |
| Blood pressure | Pre-dose, 5 min, 15 min, 30 min post-dose | Systolic >90 mmHg; treat significant hypotension with position change and IV fluids |
| Heart rate | Continuous during administration | >50 bpm; treat symptomatic bradycardia with atropine |
| Naloxone availability (not a lab test) | Before any administration: must be pre-positioned | Naloxone 0.4 mg SC/IV immediately accessible; repeat dosing may be needed |
Respiratory depression is the primary acute opioid fatality risk; must be detected immediately
Hypoxia is the downstream consequence of respiratory depression; provides objective early warning
EM-2 produces central hypotension via NTS activation: risk of syncope in research subjects
Bradycardia risk with EM-2 via central autonomic pathways
Opioid reversal is mandatory safety requirement; delay is life-threatening
Rapid onset of antinociception following central (ICV or IT) administration in preclinical models. Peak analgesic effect reached within 1-2 minutes. Measurable mu-opioid receptor activation with dose-dependent responses in tail-flick and hot-plate assays.
Sustained peak analgesia. EM-1 produces stronger supraspinal antinociception; EM-2 engages additional spinal descending pathways. Cardiovascular effects (hypotension, bradycardia) measurable with EM-2. Rapid plasma degradation by DPP-IV begins immediately.
Gradual decline in analgesic effect as receptor desensitization and peptide degradation proceed. Acute tolerance (desensitization to the same agonist) begins to develop. Antinociceptive effect disappears completely within approximately 30 minutes after a single dose.
Return to baseline nociceptive thresholds. Complete plasma clearance of native peptides. CSF degradation proceeds more slowly (>2 hours for complete inactivation). Acute antinociceptive tolerance is measurable if re-dosed within this window.
Progressive tolerance development with repeated administration, though slower onset than morphine in preclinical comparisons. Physical dependence may emerge with chronic central infusion protocols. Naloxone-precipitated withdrawal observable in dependent animals.
Minutes 0 to 2 (Rapid onset, central administration): Peak analgesia hits within 1 to 2 minutes after intracerebroventricular dosing in rodent models. This is exclusively preclinical data. MOR activation is immediate and dose-dependent in hot-plate and tail-flick assays. Sedation, respiratory rate decrease, and motor impairment appear at higher doses. EM-2 produces measurable hypotension and bradycardia at this stage. No human self-administration data exists for native endomorphin. Minutes 2 to 15 (Sustained peak): Antinociception holds at its peak. EM-1 dominates supraspinally; EM-2 recruits descending pathways involving dynorphin A and met-enkephalin for broader spinal analgesia. Cardiovascular effects from EM-2 remain measurable. Sedation continues. GI motility slows. No community data. Minutes 15 to 30 (Declining effect): DPP-IV degradation and beta-arrestin-2 mediated MOR internalization pull the analgesic effect down. By 30 minutes after a single ICV dose, antinociception is gone. Residual sedation fades. Nociceptive thresholds return to baseline. Acute tolerance is already measurable if you re-dose within this window. 30 minutes to 2 hours (Back to baseline): Native peptide is fully cleared from plasma. Cerebrospinal fluid degradation is slower, taking over 2 hours for complete inactivation. Re-dosing during this period produces a blunted response due to MOR desensitization. Days 3 to 7 (Repeated dosing, tolerance window): Progressive analgesic tolerance builds with daily dosing. Onset is slower than morphine but still within the same order of magnitude. Physical dependence emerges with chronic central infusion. Dose escalation becomes necessary. Abrupt discontinuation or naloxone challenge produces observable withdrawal signs in dependent animals.
Peak analgesic effect within 1–2 minutes after ICV administration in rodent models. Dose-dependent antinociception in hot-plate and tail-flick assays. MOR activation measurable immediately.
No human self-administration data exists.
Maintained peak antinociception. EM-1 stronger supraspinally; EM-2 engages additional descending pathways via dynorphin A and met-enkephalin. Cardiovascular effects (hypotension, bradycardia) measurable with EM-2.
No data.
Gradual analgesic decline as DPP-IV degradation proceeds and MOR desensitization (beta-arrestin-2 internalization) accumulates. Antinociception disappears within ~30 minutes after a single ICV dose.
No data.
Complete plasma clearance of native peptides. CSF inactivation slower (>2 hours for complete degradation). Acute antinociceptive tolerance measurable if re-dosed within this window.
No data.
Progressive analgesic tolerance with repeated administration: slower onset than morphine but within the same order of magnitude. Physical dependence emerges with chronic central infusion. Naloxone-precipitated withdrawal signs observable in dependent animals.
No data.
Source: Plasma t1/2 estimated at 2-8 minutes; rapid degradation by DPP-IV and aminopeptidases. CSF degradation >2 hours (Fichna et al., 2007, PMID 17329549; Horvath, 2000, PMID 11337033)
Loading the interactive decay curve.
Endomorphin-1 and Endomorphin-2 have no FDA approval for any indication. They are classified as research-only compounds. No Investigational New Drug application covers native endomorphin for general research use, though CYT-1010 (a cyclized EM-1 derivative) has completed Phase I clinical testing under its own IND. These peptides are not scheduled substances under the Controlled Substances Act, but they are mu-opioid receptor agonists. Any research involving opioid peptide administration in humans requires institutional review board approval and DEA compliance review depending on jurisdiction. No consumer peptide vendor stocks endomorphin. Sourcing is limited to scientific reagent suppliers (GenScript, Bachem, Phoenix Pharmaceuticals) for in vitro and animal research. WADA has not specifically listed native endomorphins, but opioid peptides and their analogs fall under the S7 narcotics class, prohibited in competition. This content is for educational and research reference only. Nothing here constitutes medical advice or an endorsement of human self-administration.
Peptide Schedule Research TeamReviewed Apr 202611 Citations
