A mixture of fumaric acid esters (FAEs) is approved for the oral therapy of psoriasis. However, for a long time the active ingredient of this mixture was unknown. We reviewed the in vitro data available for the different FAEs present in the multi compound drug and elaborate how they may contribute to possible clinical effects. Although helpful overall, many in vitro data must be viewed critically because the concentrations used in the experiments exceed the plasma levels reached in patients. The data suggest that dimethylfumarate (DMF) is the most active compound, mediating the major therapeutic effect after metabolization into monomethylfumarate (MMF) via an according receptor expressed on target cells. Identifying the active pharmaceutical ingredient within a mixture of compounds helps to subsequently eliminate unnecessary, potentially harmful compounds. This provides a promising example for an alternative precision medicine approach in clinical practice.