Isoniazid

Introduction

Written by Lee Holland, PharmD, MPH
Medically reviewed by Victor Nguyen, PharmD, MBA
Updated on September 15, 2023

Isoniazid (eye soe NYE a zid) is a generic medication prescribed with other drugs for treating a serious infection called tuberculosis (TB). It’s prescribed by itself to help prevent TB. Isoniazid belongs to a group of drugs called antibiotics.

Boxed warning: Risk of hepatitis

Isoniazid has a boxed warning about the risk of hepatitis. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA).

For details, see the “Interactions and warnings for isoniazid” section.

Side effects of isoniazid

Isoniazid may cause mild or serious side effects. More common mild side effects of isoniazid and its serious side effects are listed below. This article doesn’t include all possible side effects of the drug. Side effects can vary based on your age, overall health, and any other medications you take.

To learn more about isoniazid’s side effects, see this article or ask your doctor or pharmacist. You can also refer to the prescribing information for isoniazid listed in the “Resources” section.

Mild side effects

More common mild side effects reported with some forms of isoniazid are listed below. These side effects may differ based on the form of isoniazid you’re prescribed.

This article doesn’t include all possible mild side effects of the drug. To learn more about isoniazid’s side effects based on the form, refer to the prescribing information for isoniazid listed in the “Resources” section.

With many drugs, mild side effects may go away within a few days or a couple of weeks. If the side effects are bothersome, tell your doctor or pharmacist. They may be able to suggest ways to manage them.

Isoniazid oral solution’s and oral tablet’s mild side effects include:

• nausea
• vomiting
• abdominal pain
• mild allergic reaction*

Isoniazid injection’s mild side effects include:

• nausea
• vomiting
• abdominal pain
• irritation where isoniazid is injected
• mild allergic reaction*

* For details about this side effect, see the “Interactions and warnings for isoniazid” section.

Serious side effects

Serious side effects of isoniazid are listed below. These side effects may differ based on the form of isoniazid you’re prescribed.

This article doesn’t include all possible serious side effects of the drug. To learn more about isoniazid’s side effects based on the form, refer to the prescribing information for isoniazid listed in the “Resources” section.

With many drugs, serious side effects are possible but not common. If you have serious side effects from this drug, call your doctor right away. If you’re having severe symptoms or a medical emergency, call 911 or a local emergency number.

In general, isoniazid’s serious side effects include:

• serious skin reactions called drug reaction with eosinophilia and systemic symptoms syndrome and toxic epidermal necrolysis, which may be life threatening
• tingling and pain in the hands and feet
• boxed warning: risk of hepatitis*
• severe allergic reaction*

* For details about this side effect, see the “Interactions and warnings for isoniazid” section.

Common questions about isoniazid

Below you’ll find answers to a few commonly asked questions about isoniazid.

What is isoniazid’s mechanism of action? How does the drug work?

Isoniazid works to treat and help prevent tuberculosis (TB) by preventing the bacteria that cause TB from making a certain acid. This stops the bacteria from surviving in your body.

It usually takes between 6 months and 1 year for the drug to cure TB. But you may begin to feel better after a few weeks of treatment. It’s very important that you continue isoniazid treatment until your doctor says it is safe to stop. If you stop treatment early, the TB won’t be cured, and it will take longer to clear the infection.

If you have questions about how isoniazid works to treat or help prevent TB, talk with your doctor or pharmacist.

Does isoniazid cause long-term side effects?

Yes, in some cases, isoniazid may cause long-term side effects. These last for a while after you’ve stopped treatment with the drug.

Examples of long-term side effects include:

• liver damage, especially in people ages 50 years and older
• enlarged breast tissue in males*

Talk with your doctor to learn more about these side effects and how to best manage them. Ask your doctor for more information about how long these side effects of isoniazid may last.

* Sex and gender exist on spectrums. In this article, we use the term “male” to refer to a person’s sex assigned at birth.

Will I experience certain side effects if I stop isoniazid?

No, you shouldn’t have any side effects if you stop isoniazid. But if you stop the drug before you finish your full treatment course, your TB may become antibiotic resistant. (This means it may be more difficult to treat it with antibiotics.)

It’s important to take or receive isoniazid and other drugs prescribed for TB according to your doctor’s directions. Be sure to ask your doctor before stopping treatment with this drug.

If you have questions about stopping isoniazid or other medications, talk with your doctor or pharmacist.

Uses of isoniazid

The Food and Drug Administration (FDA) approves prescription drugs such as isoniazid for certain conditions. Approved uses for isoniazid are described below.

Use for treating tuberculosis

Doctors may prescribe isoniazid oral tablet, oral solution, or injection* for treating tuberculosis (TB). It’s prescribed for this purpose in people of all ages. TB treatment with isoniazid usually lasts for 6 months to 1 year. Other antibiotics are prescribed with it.

TB is caused by bacteria that mainly affect the lungs. TB can also affect other parts of the body, including the spine. Common TB symptoms include cough, chest pain, weight loss, fever, night sweats, and coughing up blood. However, symptoms can vary depending on the body part affected.

There are two types of TB infections, and both require treatment. These include:

• Active TB. This means you have symptoms and the infection can spread to others.
• Latent TB. This means you don’t have symptoms or feel ill. Your immune system is able to stop TB symptoms and the spread of TB. However, you could develop an active infection later, especially if your immune system becomes weakened.

Isoniazid is prescribed together with other antibiotics to treat TB. Examples include rifampin, pyrazinamide, and ethambutol or streptomycin. It’s difficult to treat TB, so several drugs are needed to cure the infection.

* Isoniazid injection is given into the muscle by a healthcare professional.

Use for helping to prevent tuberculosis

Doctors may prescribe isoniazid oral tablet, oral solution, or injection* for helping to prevent tuberculosis (TB). (To learn about TB, see the section directly above.) It’s prescribed for this purpose in people of all ages. The usual treatment duration is 3–9 months.

Isoniazid is prescribed for this purpose in certain people who have an increased risk of getting TB or have been exposed to TB.

* Isoniazid injection is given into the muscle by a healthcare professional.

Interactions and warnings for isoniazid

Below, you’ll find information about isoniazid’s possible interactions and warnings.

Interactions of isoniazid

Certain things may affect how a drug works. These include getting vaccines, consuming alcohol or certain foods, or taking or receiving the drug with other medications. This effect is called a drug interaction.

Before you start isoniazid, ask your doctor to check for possible interactions. Be sure to tell them about any of the following you take or use:

• prescription or over-the-counter drugs
• vitamins, herbs, or supplements

To learn about drug-condition interactions, see the “Warnings for isoniazid” section below.

Warnings for isoniazid

Some people should not take or receive isoniazid, and others should take or receive it cautiously.

Boxed warning: Risk of hepatitis

Isoniazid has a boxed warning about the risk of hepatitis. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA).

With hepatitis, you have inflammation in your liver. It can lead to death if not treated. This side effect was rarely reported in studies of isoniazid.

Symptoms may include lack of appetite, nausea and vomiting, dark-colored urine, and yellowing of the skin and eyes. You may also have tiredness, rash, and tingling in the hands and feet.

Your doctor will monitor your liver function with blood tests before and during treatment with isoniazid.

If you experience symptoms of hepatitis, tell your doctor immediately. They’ll likely recommend a different drug for your condition.

To learn more, talk with your doctor or pharmacist.

Other warnings

Isoniazid could cause harm to people with certain health conditions. This effect is called a drug-condition interaction. Other factors can also affect whether isoniazid is a safe option for you. These warnings may differ based on the form of isoniazid you’re prescribed.

Ask your doctor about specific warnings for isoniazid, and be sure to tell your doctor about your:

• current health, including any allergies to medications
• past health conditions or surgeries

Allergic reaction

Isoniazid can cause an allergic reaction in some people. Severe allergic reactions are rare but possible.

If you’ve had an allergic reaction to isoniazid or any of its ingredients, your doctor will likely not prescribe isoniazid. They can tell you about medications that are safer options for you.

A mild allergic reaction may cause the following symptoms:

• skin rash
• itchiness
• flushing (temporary warmth, redness, or deepening of skin color)

A severe allergic reaction may cause the following symptoms:

• swelling under your skin, usually in your hands, feet, eyelids, or lips
• swelling of your mouth, throat, or tongue, which can cause breathing problems

If you have an allergic reaction to isoniazid, call your doctor right away. If you have severe symptoms, call 911 or a local emergency number.

Overdose of isoniazid

You should not take more isoniazid than your doctor prescribes. For some drugs, doing so may lead to harmful effects or overdose.

If you think you’ve taken too much isoniazid, call your doctor or pharmacist right away. Or you could call 800-222-1222 to speak with someone at America’s Poison Centers. You can also use its online resource. If you have concerning symptoms, call 911 or a local emergency number immediately. You can also go to the closest emergency room.

Pregnancy or breastfeeding and isoniazid

Information about isoniazid and pregnancy and breastfeeding is described below.

Isoniazid and pregnancy

In general, isoniazid is considered safe during pregnancy. That said, ask your doctor about whether they feel it’s safe for you.

If you’re planning a pregnancy or can become pregnant, talk with your doctor about tuberculosis (TB) treatment options during pregnancy. Together, you can discuss the risks and benefits of your treatment options and decide whether you should have isoniazid during pregnancy.

Isoniazid and breastfeeding

In general, isoniazid is considered safe while breastfeeding. That said, ask your doctor about whether they feel it’s safe for you.

If you’re planning to breastfeed, talk with your doctor about TB treatment options. Together, you can discuss the risks and benefits of your treatment options and decide whether you should have isoniazid during this time. Your doctor may recommend other healthy ways to feed your child.

What to ask your doctor

Let your doctor know if you have questions about isoniazid or would like more details about it.

Here’s a list of some possible questions you may want to ask your doctor:

• Should I take isoniazid on an empty stomach?
• Are there any foods I should avoid during isoniazid treatment?
• What might happen if I stop treatment with tuberculosis (TB) medications early?
• Am I at a higher risk of hepatitis* with isoniazid than other people?

* Isoniazid has a boxed warning about the risk of hepatitis. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA). For details, see the “Interactions and warnings for isoniazid” section.

Resources

• Food and Drug Administration. (2023). Orange Book: Approved drug products with therapeutic equivalence evaluations. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm
• Isoniazid injection, USP. (2022). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/77795c31-1cdf-4a20-b4a1-cd15f5b310f1/spl-doc
• Isoniazid oral solution, USP. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/8a6fa0b2-48e3-4c06-97c5-9ec3d6475a7c/spl-doc
• Isoniazid tablets, USP. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/e7314f55-7a60-4496-a59f-895f33109998/spl-doc
• Sterling TR, et al. (2020). Guidelines for the treatment of latent tuberculosis infection: Recommendations from the National Tuberculosis Controllers Association and CDC, 2020. https://www.cdc.gov/mmwr/volumes/69/rr/rr6901a1.htm?s_cid=rr6901a1_w
• Tuberculosis (TB). (2023). https://www.cdc.gov/tb/default.htm

Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.