Get emergency medical help if you have signs of an allergic reaction (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning in your eyes, skin pain, red or purple skin rash that spreads and causes blistering and peeling).
Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, flu-like symptoms, muscle aches, severe weakness, unusual bruising, or yellowing of your skin or eyes. This reaction may occur several weeks after you began using isoniazid.
Call your doctor at once if you have:
sudden weakness or ill feeling, or fever for 3 days or longer;
pain in your upper stomach (may spread to your back), nausea, loss of appetite;
dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes);
vision changes, pain behind your eyes;
confusion, memory problems, unusual thoughts or behavior;
a seizure (convulsions); or
pale skin, easy bruising or bleeding (nosebleeds, bleeding gums).
Common side effects may include:
numbness, tingling, or burning pain in your hands or feet;
nausea, vomiting, upset stomach; or
abnormal liver function tests.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Isoniazid side effects (more detail)
