Take this medication by mouth on an empty stomach (1 hour before or 2 hours after meals) as directed by your doctor. If you are using the liquid form of this medication, carefully measure the dose using a special measuring device/spoon. Do not use a household spoon because you may not get the correct dose.

If you also take antacids that contain aluminum, take this medication at least 1 hour before the antacid.

The dosage is based on your age, weight, medical condition, and response to treatment.

For the best effect, take this drug at evenly spaced times. If you are taking this medication daily, take it at the same time each day. If you are taking this medication on a weekly schedule, take it on the same day(s) of the week and at the same time each day. Mark the days on the calendar when you need to take the medication.

Continue to take this medication (and other TB medications) until the full prescribed amount is finished, even if symptoms disappear. Stopping the medication too early or skipping doses may result in a return of the infection and cause the infection to be more difficult to treat (resistant).

Your doctor may also direct you to take vitamin B6 (pyridoxine) to help prevent certain side effects (such as nerve problems) from isoniazid. Follow your doctor's instructions carefully.

Isoniazid may interact with foods containing tyramine/histamine (such as cheese, red wine, certain types of fish). This interaction may cause increased blood pressure, flushing of the skin, headache, dizziness, or fast/pounding heartbeat. Tell your doctor right away if any of these symptoms occur. Your doctor may recommend that you follow a special diet while taking this medication. Consult your doctor for more details.

Tell your doctor if your condition lasts or gets worse.