Isoniazid

💊 What is Isoniazid used for?

• •The label says isoniazid is recommended for all forms of tuberculosis caused by bacteria it can kill. Active tuberculosis must be treated with several anti-tuberculosis medicines together, because one medicine alone is not adequate treatment. The label also lists groups for whom isoniazid is used as preventive therapy, including people with HIV infection, close contacts of someone with newly diagnosed infectious tuberculosis, people whose skin test has recently turned positive, people with chest X-rays showing old healed tuberculosis, people who inject drugs, and people with certain medical conditions such as diabetes, silicosis, leukemia, or long-term steroid or immune-suppressing treatment.

How it works

How it is usually taken

• •The label says isoniazid should not be taken with food, because food significantly reduces how much of the medicine the body absorbs. The schedule depends on the treatment plan the prescriber chooses, which may be once daily or two or three times a week. For schedules given two or three times a week, the label says the doses should be given as directly observed therapy, meaning a healthcare worker watches the dose being taken. Follow the instructions from your prescriber or pharmacist.

Comes as

• •100 mg tablets
• •300 mg tablets (white, round, scored on one side)

⚠️ Isoniazid side effects

🛑 Who should not take Isoniazid, and who should be careful

The label says not to take it if

• 🚨You have had a severe hypersensitivity (allergic-type) reaction to isoniazid, including drug-induced hepatitis.
• 🚨You have had liver injury from isoniazid before.
• 🚨You have had severe reactions to isoniazid such as drug fever, chills, or arthritis.
• 🚨You have acute liver disease of any cause.

Tell your doctor before taking it if

• •Liver blood tests are needed before starting and regularly during treatment. The label says these should be checked monthly during preventive therapy, or more often when needed.
• •Tell your prescriber if you drink alcohol daily, have chronic liver disease, use injection drugs, have severe kidney problems, have nerve damage (peripheral neuropathy), have HIV, or are pregnant or recently gave birth.
• •Tell your prescriber about all other medicines you take, since the label lists several important interactions.
• •The label says the risk of liver problems is higher in people over 35, and those people need closer monitoring.
• •If you have had a severe reaction to isoniazid before, or had liver injury from it, the label says you should not take it again.

🔀 Isoniazid with other medicines, food and alcohol

The label says to discuss these with your doctor or pharmacist

• •Food: isoniazid should not be taken with food. Foods containing tyramine (such as cheese and red wine) and histamine (such as skipjack and tuna) should be avoided, because they can cause headache, sweating, palpitations, flushing, or low blood pressure.
• •Acetaminophen: the label reports a case of severe acetaminophen toxicity in a patient taking isoniazid.
• •Carbamazepine: isoniazid slows how the body breaks it down and can raise its levels. The label says carbamazepine levels should be checked before use together.
• •Ketoconazole: the label says a possible interaction may exist, and that levels of ketoconazole can drop when taken with isoniazid and rifampin.
• •Phenytoin: isoniazid may raise phenytoin levels, which may need dose adjustment to avoid toxicity.
• •Theophylline: isoniazid may raise theophylline levels, so levels should be monitored.
• •The label lists more interactions that were not included in the text provided, so ask your pharmacist for a full review.

Alcohol and food

The label says daily alcohol use increases the risk of isoniazid-related hepatitis. Isoniazid should not be taken with food. Tyramine- and histamine-containing foods, such as cheese, red wine, skipjack, tuna, and other tropical fish, should be avoided.

🤰 Isoniazid in pregnancy and breastfeeding

Pregnancy

The label lists this medicine as Pregnancy Category C. It says isoniazid caused harm to embryos in rats and rabbits, but was not shown to cause birth defects in mice, rats, or rabbits. There are no adequate, well-controlled studies in pregnant women. The label says isoniazid should be used to treat active tuberculosis during pregnancy because the benefit justifies the potential risk to the fetus. Preventive therapy is generally started after delivery. Babies born to mothers who took isoniazid should be watched carefully for side effects.

Breastfeeding

The label says the small amounts of isoniazid in breast milk do not cause toxicity in a nursing newborn, so breastfeeding should not be discouraged. The label also says those amounts are too low to protect or treat a nursing infant.

🗓️ Storage, missed doses and taking too much

Storage

The label says to store at 20 to 25 degrees C (68 to 77 degrees F). Protect from moisture and light.

If a dose is missed

The label does not say.

If too much is taken

The label says signs of overdose can appear within 30 minutes to 3 hours after taking the medicine. Early signs include nausea, vomiting, dizziness, slurred speech, blurred vision, and visual hallucinations such as bright colors and strange patterns. With a large overdose, the label describes breathing trouble, deep drowsiness progressing to coma, and severe seizures that are hard to control. The label says untreated or poorly treated large overdoses can be fatal, but most patients respond well to prompt treatment. If an overdose is suspected, seek emergency medical help right away. The treatment section in the label text provided is cut off.

🗒️ Questions to ask about Isoniazid

• Should I take this medicine with food, or on an empty stomach, based on my treatment plan?
• How often will I need liver blood tests while I take isoniazid?
• Which of my current medicines could interact with isoniazid, including acetaminophen?
• What symptoms should make me call you right away?
• Is there a safe way to drink alcohol while on this medicine?
• What should I do if I miss a dose?
• Will I need other tuberculosis medicines along with isoniazid?

❓ Isoniazid: frequently asked questions