Along with its needed effects, isoniazid may cause some unwanted or undesirable effects. People who are more likely to experience some of these effects will usually possess one or more of the following risk factor: old age, alcoholism, liver or kidney disease, seizure disorders.

Possible unwanted effects include:

• abnormal liver function tests
• nausea
• vomiting
• diarrhea
• stomach ache
• rash
• dizziness
• drowsiness
• jaundice (yellowing of the skin or eyes)
• itching
• vitamin B6 deficiency
• pain or tingling in hands or feet

Uncommon:

• increased blood sugar
• vision problems
• breast enlargement

If you suspect you are experiencing any unwanted effects, report them to your doctor or pharmacist.

Isoniazid is an antituberculosis drug that kills bacteria that causes tuberculosis. It inhibits bacterial cell wall synthesis.

An allergic reaction sometimes occurs with isoniazid (usually occurs within the first 3 to 7 weeks of therapy). Symptoms may include fever, rash, hives, swollen lymph nodes, itching and difficult breathing. Notify your doctor if any of theses symptoms occur.

Vitamin B6 (pyridoxine) deficiency can occur during treatment and your doctor may instruct you to take a supplement.

Isoniazid may cause liver problems. Immediately report symptoms of liver problems such as fatigue, weakness, nausea or vomiting, dark urine or yellowing of the skin.

Certain foods such as cheese (e.g. Swiss or Cheshire) or fish (e.g. tuna, skipjack) may cause redness or itching of the skin, hot feeling, fast or pounding heartbeat, sweating, chills or clammy feeling, headache, or lightheadedness.

You should also strictly limit the amount of alcoholic beverages while on isoniazid. Alcohol may keep this medicine from working properly and may be more likely to cause liver problems.

Drug interactions: It is important to tell your doctor and pharmacist about any prescription or over-the-counter medications you are taking to help avoid any serious drug interactions. The dose of one or both medications may need to be altered or a new drug may be prescribed. In some cases, one of the drugs may have to be discontinued. The following is a list of drugs and drug classes that have been known to interact with isoniazid.

• alcohol
• aluminum salts (e.g. antacids) - take isoniazid 1 hour before antacids
• acetaminophen (e.g. Tylenol®)
• blood thinners (e.g. warfarin)
• carbamazepine (e.g. Tegretol®)
• benzodiazepines (e.g. diazepam)
• seizure medication (e.g. phenytoin)
• rifampin
• ketoconazole (e.g. Nizoral®)
• meperidine (e.g. Demerol®)

Use is not recommended in the following situations:

• allergy to isoniazid or any component of the preparation

Caution is recommended in the following situations:

• kidney or liver disease

Use in pregnancy: Isoniazid is the drug of choice for treatment of tuberculosis in pregnancy.  Liver function tests throughout the pregnancy are recommended. Consult your doctor or pharmacist if you suspect you are pregnant.
Use while breastfeeding: Isoniazid is passed into breast milk.  No adverse effects have been reported, but there is a potential risk of nerve or liver damage. Breast-fed infants should be carefully observed for these effects. Contact your doctor or pharmacist before use.