Online health communities (OHCs) are increasingly relied upon by individuals exchanging social support for diagnoses and treatment regimens. It has been shown that social support from close relationships (e.g., family and friends) positively influence treatment adherence in offline environments, but much less is known about the online setting. In this study, we focus on how relationships established in an online breast cancer discussion board induce reciprocity (or reciprocal exchange of support) and its impact on adherence to a five-year hormonal therapy, a highly prevalent long-term treatment for breast cancers, with varying completion rates. We measure reciprocity as reciprocal responses to forum posts. We analyze a social interaction graph of over 6,000 patients and 100,000 responses to investigate reciprocity and how it is related to time active in the OHC and the tones communicated by authors in their posts (e.g., emotions, writing styles and social tendencies). We further assess if such reciprocity is associated with treatment adherence. We find the volume of the reciprocity that is positively associated with completing the five-year protocol, rather than the rate of the reciprocity or the fraction of the posts that received replies."