Biologics are effective agents for the treatment of psoriasis. The newest generation of biologics block interleukin 17 and 23. Physicians always prescribe these drugs in a fixed dose, but this may lead to under- and overdosing in some patients. Underdosing may lead to inadequate response or loss of response over time. Overdosage, on the other hand, can lead to higher risk of side effects and higher costs for the healthcare system. In daily clinical practice, physicians often tackle this real-world issue by blind trial- and- error dose modifications or switching to another biologic. In this study, we want to rationalize these dose modifications and optimize dosing based on the drug concentrations, measured in the blood of the patient (i.e. therapeutic drug monitoring). Depending on the drug concentration, the interval between injections will be lengthened or shortened with the aim to reach the required drug concentration to reach the best clinical result. The trial will be conducted in 14 Belgian hospitals where patients will be divided into 2 study groups: a group that will be advised on the dosing scheme of their biologic based on the measured drug concentration and a group that continues dosing as in daily clinical practice. We will monitor if the clinical response and quality of life remains stable. With this study, we will track drug concentrations as we believe that they can guide dosing of biologics and we hope to achieve better safety, lower healthcare expenses and higher patients' treatment satisfaction while striving for the best clinical response.