Many patients find using medicines burdensome. This paper reports the types of issues people experience with medicines, using a validated measure of medicines burden, and the factors associated with high burden. The cross-sectional study involved patients presenting prescriptions at pharmacies or awaiting appointments at GP practices or outpatient clinics, during October 2015 to December 2016. Adults using at least one regular medicine were asked to complete the Living with Medicines Questionnaire V3 (LMQ-3). The LMQ-3 contains 41 statements rated on a 5-point scale (strongly agree to strongly disagree), with higher scores indicating greater burden, plus a visual analogue scale for self-reporting of overall perceived burden (VAS-burden). For a subsample, access to their medication record was requested, facilitating calculation of the complexity of their medicine regimen using the Medicine Regimen Complexity Index (MRCI). Of 1,888 questionnaires distributed, 684 were returned (36.2%) and medication records obtained for 163. The median number of medicines respondents reported using was 4 (range 1 to 26). Two-thirds (418; 67.0%) used medicines more than once daily, 67 (10.1%) required assistance with medicines and 189 (28.3%) paid a prescription charge. LMQ-3 scores showed a strong positive relationship with VAS-burden scores (r = .547; p < 0.001). LMQ-3 and VAS-burden scores were lower in older age groups, but both increased with increasing number of medicines and dosing frequency. LMQ-3 score was positively related to MRCI score (n = 163; r = .217; p = 0.005), whereas VAS-burden was not. Older respondents reported lower burden in most domains. Higher numbers and frequency of medicines, paying prescription charges, needing support and deprivation increased burden across multiple domains. Factors strongly associated with high LMQ-3 scores were: needing support, high dosing frequency and unemployment. Interventions seeking to reduce medicines burden should consider targeting individuals who need support with using medicines, use at least four medicines, more than twice daily and/or pay prescription charges.