An anti-tuberculosis (TB) antibiotic
Isoniazid
A core antibiotic used as part of tuberculosis (TB) treatment, and to prevent active TB in some people.
What is Isoniazid?
Isoniazid is a key antibiotic used as part of standard tuberculosis (TB) treatment, usually combined with other TB medicines, and also to prevent TB becoming active in some people. It can cause nerve problems (tingling or numbness in the hands and feet), so vitamin B6 (pyridoxine) is often given alongside it to help prevent this. It can also affect the liver, so liver monitoring is needed and the full course must be completed.
Education and reference only. This is a plain-language guide to Isoniazid — 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
Isoniazid is one of the main antibiotics used in the UK to treat tuberculosis (TB), almost always combined with other TB medicines as part of a course lasting several months. It is also used on its own or with one other medicine to prevent TB from becoming active in people who carry it without symptoms (latent TB). As with all TB treatment, the full course must be completed even when symptoms have settled.
How it works
Isoniazid works by blocking the TB bacteria's ability to build the protective wall they need to survive, which kills the bacteria and helps clear the infection. In active TB it is combined with other medicines so the bacteria are attacked in different ways, improving cure rates and reducing resistance. It is especially effective against actively multiplying TB bacteria. It treats TB and does not work against ordinary infections like colds or flu.
Company & origin
Originated / developed by: A core anti-tuberculosis medicine.
A long-established antibiotic used in the UK as part of standard tuberculosis (TB) treatment and prevention.
Practical use
How to take Isoniazid
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it exactly as prescribed and complete the whole course, even after symptoms improve.
- It is usually taken on an empty stomach, before food, unless your team advises otherwise.
- Take any vitamin B6 (pyridoxine) your team prescribes, as it helps prevent nerve problems.
- Report tingling or numbness in the hands or feet, or any yellowing of the skin or eyes, promptly.
- If you miss a dose, take it as soon as you remember unless it is nearly time for the next; do not double up, and tell your team about missed doses.
Weighing it up
Advantages & disadvantages of Isoniazid
Advantages
- A highly effective, central part of curing and preventing tuberculosis.
- Works strongly against actively multiplying TB bacteria.
- Nerve side effects can often be prevented with vitamin B6 (pyridoxine).
Disadvantages
- Can cause peripheral neuropathy (tingling or numbness in the hands and feet).
- Can affect the liver and requires monitoring with blood tests.
- Requires a long course and strict adherence to be effective.
Practical use
Good to know
A key safety point is that isoniazid can affect the nerves, causing tingling, numbness or weakness in the hands and feet (peripheral neuropathy); to help prevent this, vitamin B6 (pyridoxine) is usually given alongside it, particularly in people at higher risk. It can also affect the liver, so liver blood tests are checked and you should report yellowing of the skin or eyes, dark urine, nausea or tummy pain. It is usually taken on an empty stomach for best absorption. As with all TB medicines, the full course must be completed exactly as prescribed to cure the infection and prevent resistance, even after you feel well.
Who should not take it / use with caution
- People who have had a serious reaction to isoniazid, or who have had drug-induced liver injury from it, should not take it.
- Used with great caution, or avoided, in people with significant liver disease or who drink heavily.
- Used carefully in people prone to nerve problems (such as some with diabetes or poor nutrition), with vitamin B6 cover, and reviewed in pregnancy or breastfeeding.
Monitoring
- Liver function blood tests before and during treatment.
- Symptoms of nerve problems, such as tingling or numbness in the hands and feet.
- Response of the infection and adherence to the full course.
Side effects
- Tingling, numbness or weakness in the hands and feet (peripheral neuropathy).
- Liver problems (reported as yellowing skin or eyes, dark urine, nausea or tummy pain).
- Less commonly, rashes, mood changes or, rarely, allergic reactions.
Key interactions
- Alcohol increases the risk of liver problems and is best avoided during treatment.
- It can raise the levels of some medicines, including certain epilepsy medicines, so doses may need adjusting.
- Always tell your team and pharmacist about all your medicines before starting anything new.
Available as: Tablets and an oral liquid; also combined with other TB medicines in single tablets, and an injection used in hospital.
Answers
Isoniazid: frequently asked questions
What is isoniazid used for?
It is a core antibiotic for tuberculosis (TB), usually combined with other TB medicines, and is also used to stop latent TB becoming active. It treats TB specifically and does not work against everyday infections such as colds.
Why am I given vitamin B6 with isoniazid?
Isoniazid can cause nerve problems such as tingling or numbness in the hands and feet, and vitamin B6 (pyridoxine) helps prevent this, especially in people at higher risk. Take the pyridoxine as prescribed, and report any tingling or numbness to your team.
Why are liver tests checked?
Isoniazid can occasionally affect the liver, so blood tests keep a check on it during treatment. Report yellowing of the skin or eyes, dark urine, severe nausea or tummy pain promptly, as these can be signs of a liver problem.
Can I drink alcohol while taking isoniazid?
It is best to avoid alcohol, as it adds to the risk of liver problems while you are on isoniazid. If you have concerns about cutting down, speak to your team for advice.
Why must I finish the whole course?
TB bacteria are slow to clear, so the full course is needed to cure the infection and prevent it returning in a harder-to-treat, resistant form. Keep taking it exactly as prescribed even after you feel well.
The wider class
About Anti-tuberculosis medicines
Isoniazid belongs to the anti-tuberculosis medicines class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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