4-AMINOSALICYLIC ACID (Paser) is an anti-tuberculosis drug used to treat tuberculosis in combination with other active agents. 4-AMINOSALICYLIC ACID (Paser) is most commonly used in patients with Multi-drug Resistant TB (MDR-TB) or when isoniazid and rifampin use is not possible due to a combination of resistance and/or intolerance. There are two mechanisms responsible for aminosalicylic acid's bacteriostatic action against Mycobacterium tuberculosis. Firstly, aminosalicylic acid inhibits folic acid synthesis (without potentiation with antifolic compounds). The binding of para-aminobenzoic acid to pteridine synthetase acts as the first step in the folic acid synthesis. Aminosalicylic acid binds pteridine synthetase with greater affinity than para-aminobenzoic acid, effectively inhibiting the synthesis of folic acid. As bacteria are unable to use external sources of folic acid, cell growth and multiplication slow. Secondly, the aminosalicylic acid may inhibit the synthesis of the cell wall component, mycobactin, thus reducing iron uptake by M. tuberculosis.