Women on opioid substitution treatment (WOST) are a group at risk for sexually transmitted infections (STIs) and blood-borne viruses (BBVs). This study aimed to explore the factors that are associated with sexual health risks among WOST, and the sexual transmission of STIs and BBVs. Also, to explore the opportunities for developing a sexual health service in community pharmacies in England. The design of this research was informed by the Intervention Mapping approach to design health promotion programmes. For this qualitative study, 20 WOST and 14 community pharmacists (CPs) were interviewed between October 2015 and April 2017. Four WOST and three CPs took part in focus groups that took place in September 2017. Recruitment for WOST took place in drug services and a service for sex workers. CPs were recruited through community pharmacies. Data were collected in Bath, Bristol and Midsomer Norton (South West of England). Qualitative data was analysed using Framework Analysis and Content Analysis. Findings indicated that there are multiple factors associated with STI/BBV risk among WOST. A Social-Ecological Model was developed including factors at the intrapersonal level – e.g., worthlessness – interpersonal level – e.g., trust in health professionals – organisational level – e.g., availability of sexual health services – community level – e.g., intersectional stigma – and public policy – e.g., current health policies. Using Intersectionality Theory, this research highlighted that social inequities of health are at the core of sexual health risks among WOST. Multiple public health strategies at different ecological levels need to be designed and implemented to reduce these inequities and prevent STIs and BBVs among WOST. Community pharmacy could play a key role in delivering sexual health services, providing equity and protecting the human rights of WOST. Suggestions for service design in community pharmacy are included in this thesis.