Periodontitis is characterized by a chronic, multifactorial inflammatory process driven by dysbiotic plaque biofilms. It is recognized as the most common chronic inflammatory non-communicable disease in humans. The advanced and severe forms of periodontitis have an estimated prevalence of 7.4%, while milder forms can affect up to 50% of the population. If left untreated, periodontitis can lead to tooth loss. However, it is largely preventable and treatable with mechanical non-surgical periodontal therapy. To improve periodontal healing and thus clinical attachment gain after non-surgical therapy, adjunctive bioactive formulations such as enamel matrix derivatives, sodium hypochlorite, locally delivered antimicrobials, or hyaluronic acid have recently been proposed. However, these agents are non-autologous formulations and expensive. Platelet-rich fibrin (PRF) acts as a scaffold that inhibits the early migration of epithelial cells into the periodontal tissues. Its regenerative properties are primarily due to its ability to promote angiogenesis. This ability is attributed to the 3D fibrin matrix, which can simultaneously transport several cytokines and growth factors. These include vascular endothelial growth factor (VEGF), insulin growth factor (IGF), transforming growth factor β1 (TGF-β1) and platelet-derived growth factor (PDGF). PRF further shows antibacterial properties and accelerates soft tissue healing. So far PRF is not routinely used in periodontal non-surgical therapy. The aim of this 6-month, split-mouth randomized clinical trial including 20 patients is to test whether PRF can improve the results after non-surgical therapy.