"Undetectable equals Untransmittable", or U=U, is a public health message designed to reduce HIV stigma and help communicate the scientific consensus that HIV cannot be sexually transmitted when a person living with HIV has an undetectable viral load. Between October 2020-February 2021 we conducted 11 in-depth interviews and 3 focus groups with diverse HIV/STI service providers (nurses, public health workers, physicians, frontline providers, and sexual health educators) in Ontario, Canada (n = 18). Our objective was to understand how U=U was communicated to sexual health service users in healthcare interactions. Interview questions were embedded in a larger study focused on improving access to HIV/STI testing. Transcripts were transcribed verbatim and analysed following grounded theory. Most providers emphasized the significance of U=U as a biomedical advancement in HIV prevention but had some challenges communicating U=U in everyday practice. We discovered four interrelated barriers when communicating the U=U message: (1) provider-perceived challenges with "zero risk" messaging (e.g., wanting to "leave a margin" of HIV risk); (2) service users not interested in receiving sexual health information (e.g., in order to provide "client centered care" some providers do not share U=U messages if service users are only interested in HIV/STI testing or if other discussions must be prioritized); (3) skepticism and HIV stigma from service users (e.g., providers explained how the hesitancy of some service users accepting the U=U message was shaped by a legacy of HIV prevention messages and persistent HIV stigma); and (4) need for more culturally appropriate resources (e.g., communities other than sexual and gender minority men; non-English speaking service users; that account for broader legal context). We discuss ways to overcome barriers to communicating the U=U message as well as the limitations and potential unintended consequences of U=U framings in the context of unequal access to HIV prevention and treatment.