High-quality discharge information is important to promote patient safety when patients are transferred from hospital to primary care. Patients with multiple long-term conditions are especially vulnerable to insufficient transfer of medication information, as they use many medications and have complex interactions with the healthcare services. The aim of this study was to investigate the effect of integrated medicines management provided to hospitalized multimorbid patients on the quality of the discharge medication information. Multimorbid patients ≥18 years, using a minimum of four regular medications from a minimum of two therapeutic medication classes were recruited from the Internal Medicine ward, Oslo University Hospital, Norway, from August 2014 to March 2016 and randomly assigned, 1:1, to the intervention or control group. Intervention patients received integrated medicines management throughout the hospital stay. Control patients received standard care. This paper reports the results of a pre-specified secondary endpoint analysis of a randomized controlled trial; the difference between the intervention and control group in the quality of discharge medication information. The analysis population comprised 384 patients. Integrated medicines management improved the discharge summary quality score of the medication information from 5.8 ± 1.5 to 8.6 ± 2.6 (mean difference 2.7, 95% CI 2.3-3.2, p < 0.001). In total 171 intervention patients (89%) received a patient discharge information letter, compared to 66 control patients (35%), p < 0.001. The quality score of the medication information in the patient discharge information letter was improved from 6.0 ± 1.8 in the control group to 10.0 ± 1.3 in the intervention group (mean difference 4.0, 95% CI 3.6-4.4, p < 0.001). Integrated medicines management delivered to multimorbid patients during a hospital stay improved the quality of the medication information in discharge summaries as well as patient discharge information letters and ensured that a discharge information letter in lay language was prepared for almost all patients who were involved in the management of their medications after discharge.