Synthetically derived peptide that is used in a wide range of skincare and makeup products, especially those claiming to have a muscle-relaxing effect similar to Botox injections. These claims typically have to do with relaxing muscle contractions when making facial expressions, thus reducing the appearance of expression lines.
If acetyl hexapeptide-8 really worked to relax facial muscles, it would work all over the face (assuming you’re using the products as directed). If all the muscles in your face were relaxed you’d have sagging, not youthful, skin, not to mention that it would affect the hand and fingers you use to apply it, which would inhibit you from picking up a cup or holding the steering wheel of your car. For all the fear espoused by companies that feature this peptide in their “works-like-Botox” products, there is considerably more efficacy, usage, and safety documentation available for Botox. 
Despite claims being made for acetyl hexapeptide-8 (argireline), there is a clinical study revealing that this ingredient is not even remotely as effective as Botox in reducing wrinkles. 
It is also interesting to note that Botox, applied topically to skin, has no impact on skin or muscles in any way, shape, or form.  Still, like all peptides, acetyl hexapeptide-8 has water-binding properties and theoretical cell-communicating ability. It’s not a throwaway ingredient, but neither is it as miraculous as the manufacturer would lead you to believe.
- Trindade de Almeida A, Carruthers J, Cox S, Goldman M, Wheeler S, Gallagher C. Patient satisfaction and safety with aesthetic onabotulinumtoxinA after at least 5 years: a retrospective cross-sectional analysis of 4;402 glabellar treatments.. Dermatol Surg.. 2015;41(S1):S19-28.
- Blanes-Mira C, Clemente J, Jodas G, Gil A, Fernández-Ballester G, Ponsati B, Gutierrez L, Pérez-Payá E, Ferrer-Montiel A. A synthetic hexapeptide (Argireline) with antiwrinkle activity. Int J Cosmet Sci. 2002;24(5):303-10.
- Lupo M, Cole A. Cosmeceutical peptides. Dermatol Ther. 2007;20(5):343-9.