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# 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](https://durgical.com/en/pharmacology/classes/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.

- PAS enters the folate pathway in place of PABA
- Its products block later folate steps
- 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 →](https://durgical.com/en/conditions/tuberculosis/)

## Adverse effects

### Common

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

### Serious (rare, important)

- Haemolysis in G6PD deficiency
- Liver injury from its acetylated metabolite
- Blood count abnormalities with hypersensitivity

## 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

## Memory aid

PAS: a PABA look-alike that Pains the stomach.

## 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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## Related drugs

- [Ethionamide](https://durgical.com/en/pharmacology/ethionamide/)
- [Cycloserine](https://durgical.com/en/pharmacology/cycloserine/)
- [Dapsone](https://durgical.com/en/pharmacology/dapsone/)

## Related conditions

- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)

[← Back to the class: Antituberculosis drugs](https://durgical.com/en/pharmacology/classes/antituberculosis-drugs/)

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](https://www.who.int/publications/i/item/9789240107243) (World Health Organization (WHO), 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J04AA01) (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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/aminosalicylic-acid/