Also called PAS, an older medicine for drug-resistant tuberculosis
Para-aminosalicylic acid
An older antibiotic used as part of combination treatment for drug-resistant tuberculosis.
What is Para-aminosalicylic acid?
Para-aminosalicylic acid, often called PAS, is an older antibiotic used as part of combination treatment for drug-resistant tuberculosis (TB), where the usual medicines no longer work well. It is always used together with other TB medicines, never on its own, to help stop resistance developing. It is taken by mouth and is well known for upsetting the stomach. It can also affect the thyroid gland and, less commonly, the liver, and it can cause allergic reactions, so it is used under specialist supervision with regular monitoring.
Education and reference only. This is a plain-language guide to Para-aminosalicylic acid — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Para-aminosalicylic acid (PAS) is a long-established antibiotic that works against the bacteria causing tuberculosis. Today it is used mainly as one of the medicines in a combination regimen for drug-resistant TB, where the bacteria are no longer killed by the first-choice treatments. It is always given alongside other TB medicines, because using any single TB medicine on its own allows resistance to build up. It is taken by mouth, often as granules, and is prescribed and supervised by a specialist TB service over a long course of treatment.
How it works
Para-aminosalicylic acid interferes with a process the tuberculosis bacteria need in order to grow, which helps stop the infection spreading and supports the other medicines in clearing it. Because TB bacteria can become resistant to a single medicine, several medicines are always used together so the bacteria are attacked in more than one way at once. PAS adds to this combined effort, which is why it is one component of a longer drug-resistant TB regimen rather than a stand-alone treatment. TB treatment takes many months, and taking every medicine consistently is essential to cure the infection and prevent further resistance.
Company & origin
Originated / developed by: Specialist manufacturer.
An older medicine used in the UK as part of combination treatment for drug-resistant tuberculosis, usually under specialist care.
Practical use
How to take Para-aminosalicylic acid
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth exactly as prescribed, always together with your other tuberculosis medicines.
- Take it with or after food, or as your specialist advises, to reduce stomach upset.
- Do not take it on its own, as using a single TB medicine alone lets resistance develop.
- Complete the full, long course even when you feel well, to cure the infection and prevent resistance.
- Report any rash, yellowing of the skin or eyes, or feeling generally unwell to your specialist.
Weighing it up
Advantages & disadvantages of Para-aminosalicylic acid
Advantages
- A useful additional option for combination treatment of drug-resistant tuberculosis.
- Taken by mouth, so it can be used as part of a home treatment regimen.
- A long-established medicine with decades of experience in TB care.
Disadvantages
- Very commonly causes stomach upset such as nausea, tummy pain and diarrhoea.
- Can affect the thyroid gland and, less commonly, the liver, so monitoring is needed.
- Must always be taken with other medicines as part of a long treatment course.
Practical use
Good to know
The most important practical point is that PAS very commonly upsets the stomach, with nausea, tummy pain and diarrhoea, especially early on; taking it with food or a soft acidic food, as advised, can help, and these effects often ease with time. It must always be taken with the other TB medicines and the full course completed, even when feeling better, because stopping early or taking it alone lets the infection return and resistance grow. Two further safety points are that it can affect the thyroid gland over time, sometimes causing an underactive thyroid, which blood tests pick up, and that it can occasionally affect the liver or cause allergic reactions. Tell your specialist about jaundice, a rash, or feeling generally unwell. Regular monitoring is part of treatment.
Who should not take it / use with caution
- People who have had a serious allergic reaction to para-aminosalicylic acid should not take it.
- It is used with caution in people with existing liver or kidney problems, under specialist guidance.
- It is used with care in people with thyroid problems, with thyroid monitoring during treatment.
Monitoring
- Thyroid blood tests during treatment, as it can cause an underactive thyroid.
- Liver function tests and watching for signs of liver problems such as jaundice.
- Reviewing how well the infection is responding and how the stomach is tolerating treatment.
Side effects
- Stomach upset such as nausea, tummy pain, wind and diarrhoea, especially early in treatment.
- An underactive thyroid over time, which thyroid blood tests are used to detect.
- Less commonly, liver inflammation (hepatitis) or allergic reactions such as a rash or fever, which need review.
Key interactions
- It can interact with other medicines, so tell your specialist everything you take.
- It may affect how some other medicines are absorbed, so timing may need to be spaced out.
- It is used as part of a combination, and the whole TB regimen is reviewed together for interactions.
Available as: Granules or tablets taken by mouth.
Answers
Para-aminosalicylic acid: frequently asked questions
What is para-aminosalicylic acid used for?
It is an older antibiotic used as part of combination treatment for drug-resistant tuberculosis, where the usual TB medicines no longer work well.
Why must it be taken with other medicines?
Using a single TB medicine on its own lets the bacteria become resistant, so several medicines are always used together, with PAS as one part of the combination.
Why does it upset my stomach so much?
Stomach upset such as nausea, tummy pain and diarrhoea is very common, especially early on; taking it with food as advised can help, and it often eases with time.
Can it affect my thyroid?
Yes, over time it can cause an underactive thyroid, which is why thyroid blood tests are part of the regular monitoring during treatment.
Can I stop it once I feel better?
No. TB treatment lasts many months, and stopping early or taking it alone can let the infection return and resistance grow, so finish the full course as prescribed.
The wider class
About Anti-tuberculosis antibiotic (older, second-line)
Para-aminosalicylic acid belongs to the anti-tuberculosis antibiotic (older, second-line) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF
- NICE CKS
Building a medicines information resource?
We create evidence-led, dose-free drug and formulary references for teams.