Charles Williams Flexner is an American physician, clinical pharmaceutical scientist, academic, author and researcher.

[1] Flexner's research currently focuses on the use of long-acting and extended-release drugs and formulations for the treatment and prevention of HIV and other infectious diseases, including tuberculosis and viral hepatitis.

He founded and directs the Long Acting/Extended Release Antiretroviral Research Resource Program (LEAP),[5] In his studies regarding treatment of HIV, Flexner was an early advocate of use of ritonavir to enhance the pharmacokinetics of other drugs.

[6] As co-founder of the CADO Conferences, he has promoted the optimization of dose and formulation of antiretroviral drugs for more cost-efficient delivery in resource-limited settings.

Results of this study highlighted the potential importance of the pharmacokinetics and tolerability of antiretroviral drugs as the HIV-infected population ages; older individuals comprise an increasing proportion of people living with HIV and AIDS.