Porous silicon (PSi) nanoparticles' tunable properties are facilitating their use at highly challenging medical tasks such as peptide delivery. Because of many different mechanisms that are affecting the interaction between the peptide and the particle, the drug incorporation into the mesoporous delivery system is not straightforward. We have studied the adsorption and loading of incretin hormone glucagon like peptide 1 (GLP-1) on PSi nanoparticles. The results show that the highest loading degree can be achieved in pH values near the isoelectric point of peptide, and the phenomenon is independent of the surface's zeta potential. In order to study the interaction between the peptide and the nanoparticle, we studied the adsorption with lower concentrations and noticed that also non-Coulombic forces have a big role in adsorption of GLP-1. Adsorption is effective and pH-independent especially on low peptide concentrations and onto more hydrophobic nanoparticles. Reversibility of adsorption was studied as a function of buffer pH. When the loading is compared to the total mass of the formulation, the loading degree is 29%, and during desorption experiments 25% is released in 4 h and can be considered as a reversible loading degree. Thus, the peptides adsorbed first seem to create irreversibly adsorbed layer that facilitates reversible adsorption of following peptides.