PAS, Para-aminosalicylic acid, commonly known as PAS, is an antibiotic used to treat tuberculosis. Aminosalicylic acid was introduced to clinical use in 1948. It was the second antibiotic found to be effective in the treatment of tuberculosis, after streptomycin. PAS formed part of the standard treatment for tuberculosis prior to the introduction of rifampicin and pyrazinamide.

Its potency is less than that of the current five first-line drugs (isoniazid, rifampicin, ethambutol, pyrazinamide, and streptomycin) for treating tuberculosis and also cost higher, but it is still useful in the treatment of multidrug-resistant tuberculosis. PAS is always used in combination with other anti-TB drugs.

Source: Wikipedia