General Info:

Isoniazid, also known as INH, belongs to the class of medicines called antimycobacterials. These medicines prevent or treat infections caused by a certain type of bacterium, including the bacterium that causes tuberculosis (TB).
Isoniazid is approved by the FDA for the prevention and treatment of TB in people with HIV/AIDS, either by itself or in combination with other antimycobacterials.

Dosage:

Isoniazid comes in oral (syrup or tablet) and injectable forms. The oral forms should be taken either 1 hour before or 2 hours after a meal, but may be taken with food if necessary to avoid gastrointestinal distress. Aluminum hydroxide-containing antacids decrease absorption of isoniazid, so isoniazid must be taken at least 1 hour before the antacid.
Isoniazid is available combined with other anti-TB medicines into one pill: with rifampin in a capsule called Rifamate, or with rifampin and pyrazinamide in a tablet called Rifater. Another drug, pyridoxine, is usually given together with isoniazid to help prevent some of the nervous system side effects caused by isoniazid.

Possible Side Effects:

Along with its desired effects, isoniazid can cause some unwanted effects, including hepatitis (dark urine, yellow eyes or skin); hepatitis prodromal symptoms (loss of appetite, nausea or vomiting, unusual tiredness or weakness); peripheral neuritis (clumsiness or unsteadiness; numbness; tingling; burning; or pain in hands and feet); seizures; mood or mental changes; skin rash or sores; fever; unusual bleeding or bruising; and blurred vision or loss of vision, with or without eye pain. Individuals should tell a doctor if they have any of these symptoms. Other side effects may not be serious and may lessen or disappear with continued use of this medicine. Less serious side effects of this medicine include stomach pain, diarrhea, nausea, and vomiting. Individuals should tell a doctor if these side effects continue or are bothersome.