The goals of this clinical study are to look at how lenacapavir (LEN) passes through the body and to assess the safety of LEN and emtricitabine/tenofovir disoproxil fumarate (F/TDF) for pre-exposure prophylaxis (PrEP) in people who inject drugs (PWID) in the United States (US). The primary objectives of this study are to characterize the pharmacokinetics (PK) of LEN and to evaluate the safety of LEN and F/TDF for PrEP in US PWID.