Coadministration has not been studied. As para-aminosalicylic acid and tenofovir-DF are predominantly renally eliminated, including by active tubular secretion, there is potential for competition for elimination via renal transport proteins, which may lead to increased concentrations of either drug. However, a clinical study performed with the calcium salt of para-aminosalicylic acid showed an unexpected substantial reduction in tenofovir exposure. The mechanism is unclear but could relate to reduced absorption in presence of the calcium salt of para-aminosalicylic acid. Until further data are available on the mechanism on the interaction, caution is needed when coadministering tenofovir with the calcium salt of para-aminosalicylic acid.