Psoriasis is a noncommunicable complex and multifactorial immune-mediated inflammatory disease, occurring in the skin and often involves joints. The most common form of psoriasis (90% of cases) is psoriasis vulgaris (PsV, also known as plaque psoriasis), resulting in inflamed itchy skin, mostly manifesting on the scalp, elbows, and knees. Approximately 30% of PsV patients develop psoriatic arthritis (PsA), chronic inflammatory arthritis, where in addition to the skin manifestation, joints are also affected with pain, swelling and stiffness. In about 80% of PsA patients, arthritis manifestation follows to the PsV development. These diseases are incurable, and the aim of the treatment is symptom remission. Novel systemic drugs, including biologic drugs targeting immunomodulating cytokines and the phosphodiesterase 4 inhibitor apremilast, can be used for the treatment of moderate to severe symptoms of these diseases. Since PsV and PsA are considered as chronic diseases, long-term treatment is necessary to maintain efficacy. The real-world evidence on the treatment patterns of biologics and apremilast in the Nordic countries is limited. The main aim of this retrospective, population- and register-based cohort study is to investigate the persistence and adherence of drugs dispensed from community pharmacies for patients with moderate to severe psoriasis vulgaris. The study will include persons who initiate treatment with adalimumab, brodalumab, etanercept, ustekinumab, secukinumab, ixekizumab, guselkumab, certolizumab pegol, tildrakizumab, or apremilast in Finland and Sweden during the study period of 2008-2020. The treatment patterns of interest include treatment gaps and restarts as well as switches during a follow-up period of up to 13 years. The study will be based on nationwide registers.