When seeking advice online about health concerns, forums dedicated to medical themes are increasingly becoming an appreciated source of information for many individuals. In online health communities, patients can ask questions or otherwise seek advice that is particularly relevant to them. While they may find some of the advice useful, other advice may be perceived as less valuable. By studying the advice-seeking, advice-giving, and advice-evaluation behaviours in one of the largest online

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... h communities in Europe, this paper looks at what determines which advice is perceived as helpful, and why. Drawing on network theory, we analysed the interaction data of 108,569 users over twelve consecutive years based on all publicly available information of an established Q&A online health community. Utilising zero-inflated negative binominal modelling, our results show that advice received from others, who have similar predominant interests, is valued more when reaching out for lay expertise. If this advice is given by peers, who can also draw on expertise from other health areas, allowing for a combination of diverse "lay" expertise, the advice is valued even more. Advice provided by those who are quick to obtain the latest knowledge available in the larger community further reinforces these effects. Our findings offer an original view to understand the influence of lay expertise exchanged via online health communities and hold implications for both policy-makers and medical practitioners regarding their approach to patient-initiated use of social media for health-related reasons.