5 Isoniazid Side Effects You Should Know About

Isoniazid is an oral antibiotic that's used to treat and prevent tuberculosis. It's sometimes referred to simply as "INH."

Combined with other medications, isoniazid is a first-choice treatment option for tuberculosis. And it's important to take this medication as prescribed if it's recommended for you. Not taking isoniazid consistently could make your infection harder to treat.

Since isoniazid is an important medication, knowing what side effects are possible can help you manage them effectively. Although experiencing isoniazid side effects isn't desirable, there are a few things you can do to help prevent or treat them.

Isoniazid side effects at a glance

While many isoniazid side effects are mild and can be managed at home, some can be severe. If you have symptoms of a severe side effect, you may need to seek immediate medical care.

Mild isoniazid side effects may include:

• Tingling and numbness in your hands and feet (peripheral neuropathy)

• Increased liver enzyme levels

• Nausea and vomiting

• Upset stomach

• Mild itchiness and rash

• Fever

• High blood sugar

• Breast tenderness and enlargement (gynecomastia)

• Fever

Rare but serious isoniazid side effects include:

• Severe nerve problems, leading to eye damage, memory problems, and abnormal muscle contractions

• Severe rashes

• Confusion, trouble concentrating, and memory problems

• Changes in blood cell counts

• Drug-induced lupus

• Pancreatitis

This medication also has a boxed warning for severe liver inflammation, called hepatitis. A boxed warning is the strict warning the FDA can apply to a medication. While rare, hepatitis from isoniazid can be life-threatening.

Here, we'll go into five isoniazid side effects in more detail.

1. Tingling in hands and feet

Nerve problems are one of the most common isoniazid side effects. The most frequent nerve problem is a condition called peripheral neuropathy. This feels like tingling and numbness in your feet and hands. Isoniazid causes peripheral neuropathy because it lowers levels of pyridoxine (vitamin B6) in the body. That's why your prescriber may recommend taking a pyridoxine supplement along with isoniazid.

Peripheral neuropathy is more likely with higher doses of isoniazid. It's also more likely if you have certain health conditions, such as diabetes. And if you have malnutrition, a serious condition when you don't get enough nutrients, or you drink a lot of alcohol, you're also at increased risk.

If you experience tingling sensations in your hands, feet, or elsewhere while taking isoniazid, let your prescriber know immediately. In some cases, peripheral neuropathy can be permanent. So it's important to let your prescriber know as soon as you experience it. They may prescribe pyridoxine or increase your dose if you're already taking it.

Other nerve and brain-related problems can also occur with isoniazid. But these are more rare than peripheral neuropathy. This may include memory problems, abnormal muscle contractions (convulsions), and eye damage. If you notice any of these serious symptoms, let your healthcare team know right away. If your symptoms seem life-threatening, call 911 or go to the nearest emergency room.

2. Liver damage

Liver problems are another possible isoniazid side effect. These problems can range from mild (like mildly increased liver enzymes and jaundice) to severe liver damage (like life-threatening hepatitis). Increased liver enzymes can occur in up to 20% of people taking isoniazid. It usually happens in the first 3 months of treatment although it can occur anytime (even after you're done taking isoniazid). In most cases, liver enzyme levels return to normal without treatment.

If you're at higher risk for severe liver damage, your prescriber may recommend against taking isoniazid. People at greater risk include those who drink alcohol daily, have liver or kidney disease, or use injection drugs. Adults over 35 years old are also at increased risk of liver damage.

Your prescriber will check your liver function before, during, and after treatment with isoniazid. If your liver enzymes are extremely elevated, your prescriber may recommend stopping isoniazid and trying another tuberculosis medication.

Symptoms of liver problems may not appear right away. But they can include nausea, fatigue, and stomach discomfort. Flu-like symptoms, dark urine, and unexplained weight loss are also possible. Let your prescriber know right away if you have these symptoms. And if symptoms feel severe, seek emergency care.

3. Gastrointestinal side effects

Isoniazid can also cause gastrointestinal (GI) side effects, such as nausea, vomiting, and upset stomach. Mild symptoms are often manageable at home.

With some medications, taking it with food can help these symptoms. But this isn't recommended with isoniazid. It's best taken on an empty stomach to make sure your body absorbs it completely.

To manage GI side effects, try staying hydrated and eating smaller but more frequent meals throughout the day. You can also try avoiding spicy or fried foods. If your symptoms aren't going away or are getting worse, let your healthcare team know. They may recommend a medication like calcium carbonate (Tums) or pantoprazole (Protonix) to help. They may also want to check for signs of liver damage, since GI side effects can be a sign of liver problems.

4. Rash

Experiencing a rash and itchiness are also possible with isoniazid. If no other symptoms are present, such as fever or blisters, it may be treatable with antihistamines. Examples includecetirizine(Zyrtec) or diphenhydramine (Benadryl).

However, severe rashes can also be a side effect of isoniazid. Examples of severe rashes include:

• Steven-Johnson's Syndrome (SJS)

• Toxic epidermal necrolysis (TEN)

• Drug reaction with eosinophilia syndrome (DRESS)

Other tuberculosis medications can cause a rash too. This includes another common antibiotic for tuberculosis called rifampin (Rimactane).

Serious rashes usually occur with other symptoms, such as fever, swollen lymph nodes, and liver damage. These rashes often require emergency medical treatment. They can be life-threatening if not treated quickly.

If you experience any type of rash with isoniazid, contact your prescriber right away. They can help you determine if your rash is mild and can be treated at home. Or you may need to stop taking isoniazid and seek additional care if they think your rash is severe. If at any point you have more concerning symptoms - like a rash that comes on suddenly, blistering, or a rash with fever - seek emergency care.

5. Blood cell level abnormalities

Isoniazid can affect blood cell levels in the body. It can cause low levels of red blood cells (anemia), platelets (thrombocytopenia), and white blood cells (leukopenia) among others. You may not notice any symptoms of low blood cell levels. Or you may feel more tired or weak than usual. You may also be more likely to get infections.

Your prescriber may ask you to complete blood tests to monitor your blood cell levels. If they're low, you may need to stop taking isoniazid. But don't make any changes to your medication without talking to your prescriber first. And let them know if you feel unusually weak or tired.