The macrocyclic host cucurbit[7]uril forms very stable complexes with the diprotonated (KCB[7]1 = 1.8 × 108 dm3 mol−1), monoprotonated (KCB[7]2 = 1.0 × 107 dm3 mol−1), and neutral (KCB[7]3 = 1.2 × 103 dm3 mol−1) forms of the histamine H2-receptor antagonist ranitidine in aqueous solution. The complexation behaviour was investigated using 1H NMR and UV–visible spectroscopy as a function of pH and the pKa values of the guest were observed to increase (ΔpKa1 = 1.5 and ΔpKa2 = 1.6) upon host–guest complex formation. The energy-minimized structures of the host–guest complexes with the cationic guests were determined and provide agreement with the NMR results indicating the location of the CB[7] over the central portion of the guest. The inclusion of the monoprotonated form of ranitidine slows the normally rapid (E)–(Z) exchange process and generates a preference for the (Z) isomer. The formation of the CB[7] host–guest complex greatly increases the thermal stability of ranitidine in acidic aqueous solution at 50 °C, but has no effect on its photochemical reactivity.