Scope: The purpose of this study is to characterize the pharmacokinetics (PK) of understudied drugs administered to lactating women receiving antiretroviral and other related drugs per standard of care as prescribed by their treating caregiver. In order to understand drug transfer into breastmilk and determine subsequent infant exposure, biological samples will be collected from lactating women (blood and breastmilk) and infants (blood). The study design (“Opportunistic design”) of this study will allow for a minimal risk study, an expanded enrollment net, evaluation of off-patent drugs, and capitalization on procedures performed per standard of care to maximize study efficiency and data collection and minimize potential harm to participants. The data collected through this initiative will provide valuable PK, dosing, and safety information for drugs in vulnerable population.