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Pharmacology · Antituberculosis drugs

Aminosalicylic acid !

One of the oldest TB drugs (PAS), a PABA look-alike that disrupts the bacillus's folate pathway. Stomach upset limits its use.

Updated 2026-11-01

  • Class: Antituberculosis drugs
  • Enzyme: Folate pathway of M. tuberculosis (acts downstream of dihydropteroate synthase)
  • ATC code: J04AA01
  • WHO Model List of Essential Medicines

Also called: Para-aminosalicylic acid, PAS, 4-aminosalicylic acid

Mechanism of action

PAS is a structural analogue of para-aminobenzoic acid (PABA). It was long thought to compete at dihydropteroate synthase, but it behaves as a prodrug whose products block the folate pathway further down, starving the bacillus of the folate it needs to make DNA. It is bacteriostatic and acts only on M. tuberculosis.

  1. PAS enters the folate pathway in place of PABA
  2. Its products block later folate steps
  3. Without folate the bacillus stops multiplying

Pharmacokinetics

Absorption
High oral bioavailability; food increases it and reduces stomach irritation
Metabolism
Liver N-acetylation to a potentially hepatotoxic metabolite
Elimination
Mostly urine; reduced in renal failure

Indications

  • Multidrug- or rifampicin-resistant tuberculosis, only when bedaquiline, linezolid, clofazimine or delamanid cannot be used Tuberculosis →

Adverse effects

Common

  • Nausea, vomiting, diarrhoea, abdominal pain
  • Hypersensitivity: rash, fever, eosinophilia

Contraindications

  • G6PD deficiency
  • Hypersensitivity to aminosalicylic acid

Key interactions

Food
Increase absorption and reduce stomach irritation; it is given after meals

What is monitored

  • Gastrointestinal tolerance and adherence
  • Liver tests
  • Kidney function
  • Signs of hypersensitivity

Administration and safety

  • By mouth after meals, divided through the day
  • Protected from light and moisture: it turns brown or purple when exposed

Nursing care

  • Ask about nausea and diarrhoea, the main cause of stopping
  • Check the G6PD status
  • Teach to report rash or fever early
  • Check storage: report discoloured granules to the pharmacy

Test yourself

Think of the answer, then tap the question to see it.

What molecule does PAS resemble?

Para-aminobenzoic acid (PABA), the folate pathway substrate.

Which adverse effect most limits PAS?

Gastrointestinal intolerance: nausea, vomiting and diarrhoea.

In whom is PAS avoided because of haemolysis?

People with G6PD deficiency.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: WHO consolidated guidelines on tuberculosis: module 4: treatment and care (World Health Organization (WHO), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 65 (McGraw Hill, 2023).

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Representative model, for educational purposes only.