Abstract Introduction Sennosides are the main active constituents of the dried leaves and/or pods of Senna alexandrina Mill. that are used as laxatives. A hypothesis is that aglycones are formed during the degradation of sennosides. However, it is unknown, whether this happens under visible light exposure and how photosensitive sennosides behave in solution. Objectives Pure anthraquinone glycosides were tested on their behaviour during sample preparation in the lab under visible light exposure in dependence on the instability of the solvent. Materials and methods Samples before and after exposure were analysed using UHPLC with UV/Vis and MS detection. Results Under visible light protection, the solutions were stable for 14 days at room temperature whereas a loss of 20%–60% was measured after 1 day of light exposure. The loss of sennosides due to degradation can be as fast as up to 2%–2.5% per hour, which might have a tremendous impact on phytochemical analysis results during the course of an analysis. The formation of aglycones was not observed in the degradation of sennosides and rhein‐8‐O‐glucoside. Conclusion Aglycones could not be found as a result of the forced degradation. The solutions of sennosides clearly need to be protected from light to obtain reliable analytical results, and light protection is a major point for the stability of liquid preparations.