For Singulair® granules, the AUC0–4h was significantly lower for the direct administration scenario compared with when the granules were mixed with vehicles, except for orange juice, blackcurrant squash and applesauce UK. For the co-administration with drinks scenario, drug dissolution was higher at 4 h when the formulation was mixed with milk (61.9%), followed by when it was mixed with formula, orange squash, blackcurrant squash and orange juice (percentage of drug dissolved = 41.1, 16.7, 10.8 and 7.7, respectively). For an ionisable compound like montelukast (amphoteric; pKa basic 2.7 and pKa acidic 5.8 (34)), an increase in pH can affect the ionisation percentage of the drug. Therefore, drug solubilisation and dissolution are higher when the formulation is mixed with a dairy drink (pH between 6.5 and 6.8 (11)) in comparison with other vehicles due to an increase in the drug ionisation percentage. For co-administration with soft foods, the highest drug dissolution was observed when the granules were mixed with plain yoghurt (39.3%) and the lowest when the formulation was mixed with applesauce UK (6.4%). Drug dissolution differed when vehicles of the same subtype were tested (AUC0–4h differed between milk/formula, and between squashes, yoghurts and applesauces; p < 0.05). The pH of the dissolution media at 4 h was 6.8 ± 0.15 for all tested scenarios. The lower drug dissolution observed when the granules were mixed with applesauce UK, in comparison with when mixed with the other soft foods, is probably due to the presence of starch in its composition, which forms a net gel around the formulation that is strengthened by fruit pieces and negatively impacts drug dissolution (13). Results show that vehicles of the same type (e.g. soft foods) have a distinct impact on drug dissolution (e.g. extremely low to no drug dissolution in the case of mixing with applesauce UK but not when formulation was mixed with plain yoghurt) and it can be hypothesised that this vehicle-impact may, ultimately, affect drug behaviour in vitro. This is of particular importance considering that the recommendations for administration of Singulair® granules are to mix with ‘a spoonful of cold soft foods’ (26). Therefore, the differences observed in drug dissolution indicate the potential risk of not following vehicle recommendations in clinical practice. Moreover, when evaluating vehicle suitability during drug development, the physicochemical properties of the vehicles should be considered.