If you are taking isoniazid by mouth and it upsets your stomach, take it with food. Antacids may also help. However, do not take aluminum-containing antacids within 1 hour of taking isoniazid. They may keep this medicine from working properly.

For patients taking the oral liquid form of isoniazid:

• Use a specially marked measuring spoon or other device to measure each dose accurately. The average household teaspoon may not hold the right amount of liquid.

To help clear up your tuberculosis (TB) completely, it is very important that you keep taking this medicine for the full time of treatment, even if you begin to feel better after a few weeks. You may have to take it every day for as long as 6 months to 2 years. It is important that you do not miss any doses.

Your doctor may also want you to take pyridoxine (e.g., Hexa-Betalin, vitamin B ₆) every day to help prevent or lessen some of the side effects of isoniazid. This is not usually needed in children, who receive enough pyridoxine in their diet. If it is needed, it is very important to take pyridoxine every day along with this medicine. Do not miss any doses.

Dosing

The dose of isoniazid will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of isoniazid. If your dose is different, do not change it unless your doctor tells you to do so.

The number of tablets or teaspoonfuls of syrup that you take depends on the strength of the medicine.

• For oral dosage forms (tablets, syrup):
  • For preventing the return (reactivation) of tuberculosis:
    • Adults and teenagers 300 milligrams (mg) once a day.
    • Children Dose is based on body weight. The usual dose is 10 mg per kilogram (kg) (4.5 mg per pound) of body weight, up to 300 mg, once a day.
  • For treatment of tuberculosis:
    • Adults and teenagers 300 mg once a day; or 15 mg per kg (6.8 mg per pound) of body weight, up to 900 mg, two times a week or three times a week, depending on the schedule your doctor chooses for you.
    • Children Dose is based on body weight. The usual dose is 10 to 20 mg per kg (4.5 to 9.1 mg per pound) of body weight, up to 300 mg, once a day; or 20 to 40 mg per kg (9.1 to 18.2 mg per pound) of body weight, up to 900 mg, two times a week or three times a week, depending on the schedule your doctor chooses for you.
• For injection dosage form:
  • For preventing the return (reactivation) of tuberculosis:
    • Adults and teenagers 300 mg once a day.
    • Children Dose is based on body weight. The usual dose is 10 mg per kg (4.5 mg per pound) of body weight, up to 300 mg, once a day.
  • For treatment of tuberculosis:
    • Adults and teenagers 300 mg once a day; or 15 mg per kg (6.8 mg per pound) of body weight, up to 900 mg, two times a week or three times a week, depending on the schedule your doctor chooses for you.
    • Children Dose is based on body weight. The usual dose is 10 to 20 mg per kg (4.5 to 9.1 mg per pound) of body weight, up to 300 mg, once a day; or 20 to 40 mg per kg (9.1 to 18.2 mg per pound) of body weight, up to 900 mg, two times a week or three times a week, depending on the schedule your doctor chooses for you.

Missed dose

If you miss a dose of this medicine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Storage

To store this medicine:

• Keep out of the reach of children.
• Store away from heat and direct light.
• Do not store the tablet form of this medicine in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
• Keep the oral liquid form of this medicine from freezing.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For isoniazid, the following should be considered:

Allergies Tell your doctor if you have ever had any unusual or allergic reaction to isoniazid, ethionamide (e.g., Trecator-SC), pyrazinamide, or niacin (e.g., Nicobid, nicotinic acid). Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Diet Make certain your health care professional knows if you are on a low-sodium, low-sugar, or any other special diet. Most medicines contain more than just the active ingredient, and many liquid medicines contain alcohol.

Pregnancy Isoniazid has not been shown to cause birth defects or other problems in humans or animals. Studies in rats and rabbits have shown that isoniazid may increase the risk of fetal death. However, tuberculosis is a very serious disease and many women have been treated with isoniazid during pregnancy with no problems occurring in their babies.

Breast-feeding Isoniazid passes into the breast milk. However, isoniazid has not been reported to cause problems in nursing babies. Also, there is not enough isoniazid in breast milk to protect or treat babies who have been exposed to tuberculosis.

Children Isoniazid can cause serious side effects in any patient. Therefore, it is especially important that you discuss with the child's doctor the good that this medicine may do as well as the risks of using it.

Older adults Hepatitis may be especially likely to occur in patients over 50 years of age, who are usually more sensitive than younger adults to the effects of isoniazid.

Other medicines Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking or receiving isoniazid it is especially important that your health care professional know if you are taking any of the following:

• Acetaminophen (e.g., Tylenol) (with long-term, high-dose use) or
• Alfentanil (e.g., Alfenta) or
• Amiodarone (e.g., Cordarone) or
• Anabolic steroids (nandrolone [e.g., Anabolin], oxandrolone [e.g., Anavar], oxymetholone [e.g., Anadrol], stanozolol [e.g., Winstrol]) or
• Androgens (male hormones) or
• Antithyroid agents (medicine for overactive thyroid) or
• Carmustine (e.g., BiCNU) or
• Chloroquine (e.g., Aralen) or
• Dantrolene (e.g., Dantrium) or
• Daunorubicin (e.g., Cerubidine) or
• Disulfiram (e.g., Antabuse) or
• Divalproex (e.g., Depakote) or
• Estrogens (female hormones) or
• Etretinate (e.g., Tegison) or
• Gold salts (medicine for arthritis) or
• Hydroxychloroquine (e.g., Plaquenil) or
• Mercaptopurine (e.g., Purinethol) or
• Methotrexate (e.g., Mexate) or
• Methyldopa (e.g., Aldomet) or
• Naltrexone (e.g., Trexan) (with long-term, high-dose use) or
• Oral contraceptives (birth control pills) containing estrogen or
• Phenothiazines (acetophenazine [e.g., Tindal], chlorpromazine [e.g., Thorazine], fluphenazine [e.g., Prolixin], mesoridazine [e.g., Serentil], perphenazine [e.g., Trilafon], prochlorperazine [e.g., Compazine], promazine [e.g., Sparine], promethazine [e.g., Phenergan], thioridazine [e.g., Mellaril], trifluoperazine [e.g., Stelazine], triflupromazine [e.g., Vesprin], trimeprazine [e.g., Temaril]) or
• Plicamycin (e.g., Mithracin) or
• Valproic acid (e.g., Depakene) These medicines may increase the chance of liver damage if taken with isoniazid

• Carbamazepine (e.g., Tegretol) or
• Phenytoin (e.g., Dilantin) These medicines may increase the chance of liver damage if taken with isoniazid. There may also be an increased chance of side effects of carbamazepine and phenytoin

• Disulfiram (e.g., Antabuse) This medicine may increase the chance of liver damage and side effects, such as dizziness, lack of coordination, irritability, and trouble in sleeping

• Ketoconazole (e.g., Nizoral) or
• Rifampin (e.g., Rifadin) Use of these medicines with isoniazid can lower the blood levels of ketoconazole or rifampin, decreasing their effects

• Alcohol abuse (or history of) or
• Liver disease There may be an increased chance of hepatitis with daily drinking of alcohol or in patients with liver disease

• Kidney disease (severe) There may be an increased chance of side effects in patients with severe kidney disease

• Seizure disorders such as epilepsy There may be an increased chance of seizures (convulsions) in some patients