Aspartame is a phenylalanine containing sweetener, added to foods and drinks, which is avoided in phenylketonuria (PKU). However, the amount of phenylalanine provided by aspartame is unidentifiable from food and drinks labels. We performed a cross-sectional online survey aiming to examine the accidental aspartame consumption in PKU. 206 questionnaires (58% female) were completed. 55% of respondents (n = 114) were adults with PKU or their parent/carers and 45% (n = 92) were parents/carers of children with PKU. 74% (n = 152/206) had consumed food/drinks containing aspartame. Repeated accidental aspartame consumption was common and more frequent in children (p < 0.0001). The aspartame containing food/drinks accidentally consumed were fizzy drinks (68%, n = 103/152), fruit squash (40%, n = 61/152), chewing gum (30%, n = 46/152), flavoured water (25%, n = 38/152), ready to drink fruit squash cartons (23%, n = 35/152) and sports drinks (21%, n = 32/152). The main reasons described for accidental consumption, were manufacturers’ changing recipes (81%, n = 123/152), inability to check the ingredients in pubs/restaurants/vending machines (59%, n = 89/152) or forgetting to check the label (32%, n = 49/152). 23% (n= 48/206) had been prescribed medicines containing aspartame and 75% (n = 36/48) said that medicines were not checked by medics when prescribed. 85% (n = 164/192) considered the sugar tax made accidental aspartame consumption more likely. Some of the difficulties for patients were aspartame identification in drinks consumed in restaurants, pubs, vending machines (77%, n = 158/206); similarities in appearance of aspartame and non-aspartame products (62%, n = 127/206); time consuming shopping/checking labels (56%, n = 115/206); and unclear labelling (55%, n = 114/206). These issues caused anxiety for the person with PKU (52%, n = 106/206), anxiety for parent/caregivers (46%, n = 95/206), guilt for parent/carers (42%, n = 87/206) and social isolation (42%, n = 87/206). It is important to understand the impact of aspartame and legislation such as the sugar tax on people with PKU. Policy makers and industry should ensure that the quality of life of people with rare conditions such as PKU is not compromised through their action.