The encapsulation of pharmaceutically active species in porous host materials can have a variety of benefits, such as increased bioavailability, enhanced solubility of poorly soluble drugs, and prolonged drug release, which can reduce side effects and enhance therapeutic efficacy. Moreover, the use of "active" drug delivery systems, in which the drug is released under defined external conditions (e.g., at a certain pH value or in the presence of a magnetic field) can achieve a targeted release at a desired location in the body. Among many other classes of porous materials, zeolites, crystalline inorganic materials with a tetrahedral framework structure, have received considerable attention as materials for drug delivery applications. In aluminosilicate zeolites, the tetrahedral framework sites are occupied by Si and Al atoms, with a general framework composition Si1-xAlxO2 (x ≤ 0.5). The Si/Al ratio determines the framework charge and, consequently, the amount of positively charged species that are required for charge neutrality. Out of the variety of known zeolite frameworks types, zeolites having the FAU (faujasite) topology are particularly widely used in industrial processes, especially in catalysis and separation. Due to their good availability, large pore size, and the possibility to incorporate various species as extra-framework cations, they have also been the focus of many drug delivery investigations. FAU-type zeolites with Si/Al ratios below 1.5 are dubbed "zeolite X". Zeolite X samples exchanged with different cations were investigated for the storage and delivery of the anticancer drug cyclophosphamide, the antiinflammatory and cytostatic 6-mercaptopurine, the anti-osteoporosis drugs risedronate and zoledronate, and the antibiotic ciprofloxacin, among other species. More silicon-rich FAUtype zeolites are labelled "zeolite Y", they were studied, for example, as host materials for the non-steroidal anti-inflammatory drug ibuprofen and for the antibiotics sulfadiazine and isoniazid. Looking beyond drug delivery, highly siliceous zeolite Y, which possesses hydrophobic properties, could find use as adsorbent for the removal of pharmaceuticals and other organic contaminants from wastewaters. 5-fluorouracil (C4H3FN2O2, CAS No. 51-21-8, Figure , abbreviated 5-FU in the following) is a cytostatic drug that is used in the treatment of various types of cancer; it is typically administered intravenously. Due to its limited bioavailability and low absorption, high doses are required, often resulting in severe side effects. Moreover, high dosages can promote the development of chemoresistance to 5-FU, reducing its therapeutic efficacy. In order to mitigate these drawbacks, the potential applicability of various materials as 5-FU carriers has been investigated. The first investigation of 5-FU storage and release using zeolites was published by Datt et al. in 2013, who compared three different samples of protonic zeolite Y with Si/Al ratios of 2.5, 15, and 30 (in protonic zeolites, protons bonded to framework oxygen atoms balance the framework charge). While they observed similar 5-FU loadings on the order of 10 wt% for all three samples, the release behaviour was markedly different: Whereas the two more Si-rich samples released about 60% of the adsorbed 5-FU in a few minutes, the most Al-rich sample retained about 95%. On the basis of spectroscopic investigations ( 27 Al NMR, infrared), it was inferred that 5-FU forms strongly bound complexes with octahedrally coordinated aluminium (Aloct) sites. As these sites were most abundant in the most Al-rich sample, it was concluded that the limited release from this sample was caused by a strong binding of 5-FU to octahedral Al atoms. Octahedral Al sites, which show Lewis acidic properties, are often by default classified as "extra-framework aluminium" [EFAl] sites, which are fully dislodged from the framework. However, it has been established that octahedral Al atoms can occur both as EFAl sites and as "framework-associated" Al sites, which are only partially dislodged and retain some bonds to framework oxygen atoms. The relative amount of the two Aloct species varies depending on the external conditions, with harsh conditions favouring EFAl formation, and their rigorous distinction with experimental methods is not trivial. Following the study by Datt et al., a number of other groups investigated 5-FU adsorption in different zeolites including Na-Y and K-LTL, Na-X and Na-beta, Na-Y, Na-LTA, and NH4-ZSM-5, 27 protonic zeolite beta, and Ag-X. Cytotoxicity studies with human cancer cells that were performed in the context of some of these studies showed a notable increase in efficiency of zeolite-encapsulated 5-FU compared to the free drug. In one investigation, molecular dynamics (MD) simulations using classical force field parameters were used to study the diffusion of 5-FU through the pores FAU-and BEA-type zeolites. These simulations helped to rationalise the experimental observation of a slower 5-FU release from zeolite beta as compared to Na-X, as it was inferred that the tight fit of 5-FU into the pores of zeolite beta gives rise to strong van der Waals (vdW) interactions.