Tenofovir disoproxil is primarily used in the treatment of HIV infection and chronic hepatitis B. It works by inhibiting the reverse transcriptase enzyme, which is crucial for the replication of the virus. This helps in reducing the viral load in the body and improving the immune system function. It is often prescribed as part of a combination therapy to enhance its effectiveness and minimize the risk of developing drug resistance. Additionally, it is used as a preventive measure in individuals at high risk of HIV exposure, known as pre-exposure prophylaxis (PrEP), to lower the chances of contracting the virus.