Pre-exposure prophylaxis (PrEP) is a novel bio-medical HIV prevention option for individuals at high risk of HIV exposure. This qualitative interview study explores perceptions and understandings of PrEP among a sample of 20 HIV-negative and HIV-positive men who have sex with men (MSM) in the UK, where there is a debate about the feasibility of offering PrEP on the NHS. Data were analysed using qualitative thematic analysis and social representations theory from social psychology. The following three themes are discussed: (1) uncertainty and fear, (2) managing relationships with others, and (3) stigma and categorization. HIV-negative interviewees generally perceived PrEP as a risky solution for “high risk” individuals, while HIV-positive individuals regarded it as potentially enhancing interpersonal relations between serodiscordant partners. Social stigma overwhelmingly underpinned individuals’ perceptions of PrEP. This might inhibit access to PrEP among those who might benefit most from it, thereby undermining HIV prevention efforts.