Fe3O4 is frequently used as colouring pigment in medicinal products. It does not exhibit own pharmacodynamic activity and thus, it is considered as clinically inactive ingredient. Fe3O4 exhibits a dipole moment and thus, after magnetisation of solid oral dosage forms containing homogenously distributed Fe3O4, the product generates a magnetic signal, which can be used for localisation of the dosage form in the gastrointestinal tract by Magnetic Marker Monitoring technique.