With the use of fixed-dose drug combinations, the potential risk of non-adherence needs to be minimised, which is particularly important in patients with chronic diseases. The development of such dosage form is a complex process in which an important role is played by testing the physicochemical compatibility of the components intended for the manufacture of the desired product. Various analytical techniques, such as thermoanalytical (DSC, DTA), spectroscopic (FTIR, XRPD) and chromatographic (HPLC), in combination with certain degradation studies, such as forced degradation and isothermal degradation, are often used. In this review, on the example of the development of a fixed-dose combination of 5-aminosalicylate and folic acid, procedures suitable for testing the compatibility of drug components, as well as analytical techniques and various ways of interpretation of results are described.