We develop activity-based assays to measure medication concentration based on the drug’s inherent activity. One of our ongoing projects is focused on developing REverSe TRanscrIptase Chain Termination (RESTRICT) assays that measure the concentrations of nucleotide analog drugs used in HIV treatment and prevention. These molecular assays provide rapid and user-friendly measurement of HIV medications directly in patients’ blood and can be used to monitor and individualize the concentrations of oral HIV therapies and emerging long-acting injectable therapies.