Isoniazid

Treatment of Active Tuberculosis

Isoniazid is a primary medication used to treat active tuberculosis (TB). When treating active disease, it is typically used in combination with other antituberculosis drugs. Using multiple medications together helps ensure that the infection is fully cleared and reduces the risk of the bacteria becoming resistant to treatment.

Prevention of Tuberculosis

This medication is also prescribed for the prevention of TB in individuals who are at high risk of developing the disease. This use, sometimes called prophylaxis, is common for:

• People who have been in close contact with someone who has active tuberculosis.
• Individuals who have a positive tuberculosis skin or blood test but do not have active symptoms (latent TB).
• People with medical conditions or treatments that weaken the immune system, making them more susceptible to TB.

When used for prevention, isoniazid may be taken alone or in combination with other medications depending on the specific situation and medical guidance.

Targeting Tuberculosis Bacteria

Isoniazid belongs to a class of medications known as antituberculosis agents. It works by specifically targeting the bacteria that cause tuberculosis (TB). The medication is effective against bacteria that are actively dividing and can also affect bacteria that are in a resting state.

Disrupting the Cell Wall

The primary way isoniazid works is by interfering with the production of mycolic acids. These are essential components that the bacteria need to build and maintain their protective outer cell walls. When the bacteria cannot produce these acids, they are unable to form a stable cell wall, which prevents them from growing and eventually leads to their death.

Treatment and Prevention

Because of its mechanism of action, isoniazid is used in two main ways:

• Treatment: For active TB, it is usually used in combination with other medications. Using multiple drugs helps ensure all bacteria are killed and reduces the risk of the bacteria developing resistance to the treatment.
• Prevention: It may be used alone in individuals who have been exposed to TB but do not have active symptoms. In this case, it works to eliminate any bacteria present in the body before they can cause a full-blown infection.

Isoniazid is only effective against bacterial infections. It does not work for viral infections, such as the common cold or the flu.

Common Side Effects

Some people may experience mild side effects while taking isoniazid. These often include:

• Nausea or vomiting
• Stomach pain or upset
• Loss of appetite

Serious Side Effects

Contact a healthcare provider immediately if you experience any of the following serious symptoms:

• Liver problems: Yellowing of the skin or eyes (jaundice), dark urine, pale stools, unusual tiredness, weakness, or persistent nausea and vomiting.
• Nerve issues: Numbness, tingling, or a burning sensation in the hands or feet.
• Vision changes: Blurred vision or pain in the eyes.
• Mental or mood changes: Confusion, memory problems, or unusual behavior.
• Allergic reactions: Skin rash, fever, or swollen lymph nodes.
• Blood issues: Easy bruising or unusual bleeding.
• Seizures: Any new or worsening seizure activity.

Severe Liver Inflammation

Isoniazid is associated with a risk of severe and sometimes fatal liver inflammation (hepatitis). While this condition most often develops within the first three months of therapy, it can occur at any time during treatment. The risk of liver damage increases with age, particularly for individuals over 35 years old. Other factors that may increase this risk include daily alcohol consumption, chronic liver disease, and the period immediately following childbirth.

Monitoring and Symptoms

Healthcare providers typically perform regular blood tests to monitor liver function throughout the course of treatment. Monitoring for physical symptoms of liver dysfunction is also standard practice. These symptoms may include:

• Unusual fatigue or weakness
• Loss of appetite
• Nausea or vomiting
• Dark-colored urine
• Yellowing of the skin or the whites of the eyes (jaundice)
• Abdominal pain or tenderness

Nerve Damage (Peripheral Neuropathy)

Treatment with isoniazid can lead to peripheral neuropathy, which is characterized by numbness, tingling, or burning sensations in the hands and feet. This condition is more likely to occur in individuals with certain risk factors, such as malnutrition, diabetes, or alcohol use disorder. Healthcare providers often prescribe Vitamin B6 (pyridoxine) to help prevent or manage these symptoms.

Severe Skin and Allergic Reactions

Rare but serious allergic reactions and skin conditions, including Stevens-Johnson syndrome and DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms), have been reported. These conditions are medical emergencies and may present with a widespread rash, fever, and swollen lymph nodes.

Contraindications

Isoniazid is generally not used in individuals with certain medical histories, including:

• Active or acute liver disease
• A history of severe adverse reactions to isoniazid, such as drug-induced liver injury
• Previous severe hypersensitivity reactions, including drug fever, chills, or arthritis associated with the medication

Drug and Alcohol Interactions

Daily alcohol consumption while taking isoniazid significantly increases the risk of liver toxicity. Additionally, isoniazid can interact with other medications, such as the anti-seizure drug phenytoin. This interaction can lead to dangerously high levels of phenytoin in the blood, requiring dosage adjustments and close monitoring by a healthcare professional.

Pregnancy

Isoniazid crosses the placenta and reaches the fetus. It is generally used during pregnancy only when the benefits to the mother outweigh the potential risks to the unborn baby. There is some evidence of an increased risk of liver inflammation (hepatitis) in pregnant and postpartum women taking this medication.

To help prevent nerve damage, healthcare providers typically recommend taking pyridoxine (vitamin B6) supplements while using isoniazid during pregnancy.

Breastfeeding

Isoniazid is excreted into breast milk in low concentrations. Because of the potential for side effects in the nursing infant, such as liver toxicity or nerve damage, close monitoring of the baby is recommended. It is important to note that the amount of isoniazid in breast milk is not enough to treat or prevent tuberculosis in the infant.

Decisions regarding breastfeeding while taking isoniazid should be made in consultation with a healthcare provider to weigh the benefits of breastfeeding against the potential risks to the child.

Available Forms

Isoniazid is provided in several different forms to accommodate different patient needs:

• Oral Tablet: A solid medication taken by mouth.
• Oral Solution: A liquid preparation for oral use.
• Intramuscular Injection: A solution for injection into a muscle, typically administered by a healthcare professional.

Isoniazid may also be found in combination with other antituberculosis medications in tablet or capsule forms.

FDA Approval and Indications

Isoniazid is approved by the U.S. Food and Drug Administration (FDA) for the treatment of active tuberculosis (TB) and for the prevention of TB in people who are at high risk of developing the disease. When used for active TB, it is almost always prescribed in combination with other medications to prevent the bacteria from becoming resistant.

Boxed Warning

The FDA requires a boxed warning for isoniazid because it can cause severe and sometimes fatal hepatitis (liver inflammation). This condition may occur within the first few weeks of treatment or after several months. The risk of liver injury increases with age, daily alcohol consumption, and underlying liver disease. It is also reported to be higher among women, especially in the period following childbirth.

Monitoring Requirements

Due to the potential for serious side effects, regulatory standards emphasize the need for regular medical monitoring. Healthcare providers typically perform blood tests to check liver function before starting the medication and at regular intervals during treatment. Patients are advised to stop the medication and seek medical attention if they experience symptoms of liver problems, such as dark urine, fatigue, or yellowing of the eyes and skin.

Alcohol and Liver Health

It is important to avoid drinking alcoholic beverages while taking isoniazid. Combining alcohol with this medication increases the risk of severe liver damage or hepatitis.

Daily Alcohol Use

Daily consumption of alcohol while on isoniazid therapy is particularly dangerous and may lead to:

• An increased risk of drug-induced liver injury.
• Changes in how the body processes the medication, which can interfere with the effectiveness of the treatment.

Before starting isoniazid, tell your healthcare provider if you drink alcohol regularly or have a history of alcohol use disorder, as you may require closer monitoring of your liver function during treatment.