Background Type 2 diabetes mellitus (T2DM) and coronary heart disease (CHD) are two chronic diseases that cause a tremendous burden. To reduce this burden, several programmes for optimising the care for these diseases have been developed. In Germany, so-called disease management programmes (DMPs), which combine components of Disease Management and the Chronic Care Model, are applied. These DMPs have proven effective. Nevertheless, there are opportunities for improvement. Current DMPs rarely address self-management of the disease, make no use of peer support, and provide no special assistance for persons with low health literacy and/or low patient activation. The study protocol presented here is for the evaluation of a programme that addresses these possible shortcomings and can be combined with current German DMPs for T2DM and CHD. This programme consists of four components: Meetings of peer support groups Personalised telephone-based health coaching for patients with low literacy and/or low patient activation Personalised patient feedback A browser-based web portal Methods Study participants will be adults enrolled in a DMP for T2DM and/or CHD and living in North Rhine-Westphalia, a state of the Federal Republic of Germany. Study participants will be recruited with the assistance of their general practitioners by the end of June 2021. Evaluation will be performed as a pragmatic randomised controlled trial with one intervention group and one waiting control group. The intervention group will receive the intervention for 18 months. During this time, the waiting control group will continue with usual care and the usual measures of their DMPs. After 18 months, the waiting control group will also receive a shortened intervention. The primary outcome is number of hospital days. In addition, the effects on self-reported health-state, physical activity, nutrition, and eight different psychological variables will be investigated. Differences between values at month 18 and at the beginning will be compared to judge the effectiveness of the intervention. Discussion If the intervention proves effective, it may be included into the DMPs for T2DM and CHD. Trial registration The study was registered in the German Clinical Trials Registry (Deutsches Register Klinischer Studien (DRKS)) in early 2019 under the number 00020592. This registry has been affiliated with the WHO Clinical Trials Network (https://www.drks.de/drks_web/setLocale_EN.do) since 2008. It is based on the WHO template, but contains some additional categories for which information has to be given (https://www.drks.de/drks_web/navigate.do?navigationId=entryfields&messageDE=Beschreibung%20der%20Eingabefelder&messageEN=Description%20of%20entry%20fields ). A release and subsequent number assignment only take place when information for all categories has been given.

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