pentapeptide-18
[D-Ala2]-leu-enkephalin, DALA, Tyr-D-Ala-Gly-Phe-Leu, D-Ala2-leucine enkephalin
Pentapeptide-18 is a cosmetic name for [D-Ala2]-leu-enkephalin, an enzymatically stabilised analogue of the endogenous opioid peptide leu-enkephalin that has been a standard pharmacological tool since the 1970s. In skincare it is marketed as a muscle-relaxing peptide for expression lines. The opioid pharmacology is authentic and old; the cosmetic application rests on an inference about neuromuscular transmission that no published study has tested through skin.
Mechanism
Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) is one of the two classical enkephalins, agonists at delta- and mu-opioid receptors. Its usefulness as a research tool was limited by near-instantaneous degradation, so position 2 glycine was replaced with D-alanine, blocking aminopeptidase cleavage while preserving receptor affinity. The resulting molecule is the compound sold cosmetically as pentapeptide-18.
Two strands of biology are invoked. The first is presynaptic: Bixby and Spitzer showed in Nature in 1983 that enkephalin reduces quantal content at the frog neuromuscular junction (less acetylcholine released per nerve impulse), and comparable effects were later reported at the mouse neuromuscular junction. Less acetylcholine means weaker contraction, the proposed route to softened expression lines. The second is cutaneous: opioid receptors are genuinely expressed in human epidermis, and Bigliardi-Qi and colleagues showed that deleting the delta-opioid receptor in mice alters keratinocyte differentiation and delays wound healing, so opioid signalling has a real role in skin biology independent of muscle. The gap is delivery. Pentapeptide-18 is a hydrophilic, unmodified pentapeptide with no lipid conjugation. Reaching a facial neuromuscular junction requires crossing stratum corneum, viable epidermis, the full thickness of dermis and subcutaneous tissue at concentrations sufficient to occupy presynaptic opioid receptors. No published study demonstrates any part of that route, and the physicochemistry makes it improbable.
What the research shows
Bixby and Spitzer's 1983 Nature paper is the anchor for the cosmetic mechanism: enkephalin reduced the quantal content of transmitter release at the frog neuromuscular junction, consistent with presynaptic opioid receptor activation. Re and colleagues reported comparable observations at the mouse neuromuscular junction. These are genuine findings, obtained by bathing preparations in peptide at defined concentrations, not by topical application.
Where pentapeptide-18 itself has been studied, the endpoints are not muscular. Park and colleagues found that appending D-tyrosine to pentapeptide-18 conferred anti-melanogenic activity, reducing melanin content and tyrosinase activity in MNT-1 cells and primary melanocytes and reducing melanin in the basal layer of a 3D human skin model, a study using the peptide as a scaffold rather than testing its own claimed action. Akhan and colleagues characterised the peptide by FTIR, ATR-FTIR and Raman spectroscopy with density functional theory calculations, then docked it against TGF-beta, TNF-alpha, MAPK, PI3K/AKT and NF-kappaB; this is computational and spectroscopic work, and molecular docking is a hypothesis-generating exercise, not evidence of activity. Two 2024 papers formulated it into solid lipid nanocarriers with retinol. Separately, opioid signalling in skin is well established: mice lacking the delta-opioid receptor show altered epidermal differentiation and delayed wound healing, and reviews document receptor expression on keratinocytes and cutaneous nerve fibres and relevance to itch. No controlled human trial of pentapeptide-18 exists.
Evidence assessment
Preclinical only
The opioid receptor pharmacology of [D-Ala2]-leu-enkephalin is thoroughly documented, and the neuromuscular effect of enkephalins is a real published finding, but it was demonstrated by direct application to isolated nerve-muscle preparations, not through skin. The draft this record corrects claimed no PubMed paper reports the use of pentapeptide-18; that is false and was checked directly. Park and colleagues used it in human melanocytes, MNT-1 cells and a 3D human skin model, and Akhan and colleagues published a spectroscopic and computational characterisation. Neither tests muscle relaxation, and neither is a clinical study. There is still no controlled human trial, no penetration study and no independent efficacy replication; supplier claims of wrinkle-depth reduction, including for combinations with acetyl hexapeptide-8, have not been peer reviewed. Solid underlying pharmacology plus no clinical evidence for the actual use case equals preclinical.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Enkephalin reduces quantal content at the frog neuromuscular junction Preclinical only
Enkephalin reduced the number of transmitter quanta released per nerve impulse, establishing a presynaptic opioid effect on neuromuscular transmission.
Leu-enkephalin effects at the mouse neuromuscular junction Preclinical only
Reported effects of leu-enkephalin on mammalian neuromuscular transmission, extending the frog observations to a mammalian preparation.
D-tyrosine adds an anti-melanogenic effect to cosmetic peptides Preclinical only
Adding D-tyrosine to the terminus of pentapeptide-18 conferred the ability to reduce melanin content and tyrosinase activity, and reduced melanin in the epidermal basal layer of a 3D human skin model.
Pentapeptide-18 as an anti-aging candidate: Spectroscopic characterization and molecular interaction analysis Preclinical only
Characterised the peptide's vibrational spectra and electronic structure and modelled binding to TGF-beta, TNF-alpha, MAPK, PI3K/AKT and NF-kappaB targets.
Deletion of delta-opioid receptor in mice alters skin differentiation and delays wound healing Preclinical only
Loss of the delta-opioid receptor altered epidermal differentiation and delayed wound healing, showing opioid signalling has genuine functions in skin distinct from any effect on muscle.
Peripheral Opioids Preclinical only
Reviews opioid receptor expression on keratinocytes and cutaneous nerve fibres and the role of peripheral opioid signalling in skin, including itch.
Safety
No adverse effects have been reported for cosmetic use at typical concentrations. Systemic opioid effects are not a realistic concern from topical application of a small quantity of a hydrophilic pentapeptide, and enkephalins have poor oral and transdermal bioavailability. Because delta-opioid signalling influences keratinocyte differentiation and wound healing in animal models, application to broken or healing skin has not been studied and is not something the published evidence supports. There are no reproductive, developmental, pregnancy or lactation data. This is a topical cosmetic ingredient; it has not been evaluated for injection, and injectable opioid peptides are a wholly different regulatory and safety proposition.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Permitted as a cosmetic ingredient under the UK Cosmetics Regulation (retained Regulation (EC) No 1223/2009); not listed in any prohibited or restricted annex. It is not a controlled drug under the Misuse of Drugs Act 1971. A Cosmetic Product Safety Report and responsible person are required. No MHRA medicines authorisation. |
| United States | Not an FDA-approved drug and not covered by an OTC drug monograph; marketed as a cosmetic ingredient under the FD&C Act as amended by the Modernization of Cosmetics Regulation Act 2022. It is not a controlled substance; enkephalin analogues are not scheduled under the Controlled Substances Act. |
| WADA (sport) | Not named on the WADA Prohibited List. The List prohibits specified narcotics in competition, but enkephalin analogues are not among them and topical cosmetic use raises no anti-doping issue. |
Questions
It is [D-Ala2]-leu-enkephalin, Tyr-D-Ala-Gly-Phe-Leu, the natural opioid peptide leu-enkephalin with a D-alanine swapped in at position 2 to block the enzyme that would otherwise degrade it within seconds. It has been a standard opioid research tool since the late 1970s.
There is no published evidence that it does when applied to skin. Enkephalins genuinely reduce acetylcholine release at the neuromuscular junction, but that was shown by bathing isolated nerve-muscle preparations in the peptide. A hydrophilic pentapeptide reaching facial neuromuscular junctions through intact skin has never been demonstrated, and the few published studies using pentapeptide-18 itself measured pigment biology or spectroscopic properties, not muscle.
It is an opioid peptide pharmacologically, but it is not a controlled substance in the UK or the US, has negligible oral or transdermal bioavailability, and is used in tiny quantities in cosmetics. Systemic opioid effects from topical use are not a realistic concern.
Yes, though not what the cosmetic claim says. Opioid receptors are expressed on keratinocytes and cutaneous nerves, and animal work shows delta-opioid signalling influences epidermal differentiation, wound healing and itch. None of that has been studied for pentapeptide-18 specifically.